I think that it is important, even if an 1806 invasion is impossible in any circumstances. The French after Trafalgar had a significant naval construction program, which aimed to eventually rebuild the French navy to the point of being able to challenge the RN, but it wasn't anywhere near completion by the time the war ended. Without losing so many ships at Trafalgar, this can be pushed forward, and it gives the French both significantly more power during the critical 1806-1812 periods where their naval power was at a nadir, and also might steer them away from some bad decisions.
Without the loss at Trafalgar the French fleet would have had many more ships of the line, around 20 more, plus more sailors, and at least some of them must have been the experienced ones the French navy so desperately lacked. Instead of having 104 ships of the line under construction or in service in 1814 (per French Warships in the Age of Sail. 1786-1861, Design, Construction, Careers and Fates), it is ~125, with a larger portion of these being in actual service. Of course this isn't enough to challenge the Royal Navy, as England has more ships total in service and the English ships have a qualitative edge over their French opponents, but there is the eternal problem of trying to keep ships on station, since one has to deal with the wear and tear on these ships by repairs, allot times for ships to sail out and join those on station. I thought I had read somewhere it was that only 1/3 of ships can be maintained permanently on station, but even if it is lower then that means only a relative fraction of the RN can be kept in a constant blockade of the French coast. The French don't need numerical superiority over the RN to be able to have enough ships to conduct a sortie with local numerical superiority, and even if that's just for say, training, that could be very important to start fixing the qualitative problems of the French navy. Of course, the RN does have bases that help them engage in their blockades, but the basic problem persists, even if these bases alleviate it.
I also would wonder if the presence of an intact Spanish fleet - as opposed to having declined from 26 ships in 1805 to 17 in 1810, not even taking into account whatever other losses they had - might have swayed the Peninsula Campaign and prevented the situation there from turning out as it did, so badly for the French. The main Spanish benefit to the French was the Spanish fleet. When they lost the fleet, they had little remaining value. If the Spanish were able to negotiate from a better bargaining position - that they had a fleet which was of at least some marginal value - then they might be able to make the French take them more seriously, and tread more lightly in Spain and prevent the situation there from flaring up into a revolt. In addition, if the French fleet is still in Cadiz, like the POD is that the Allied fleet doesn't sail to sea with Villeneuve being so petty about the prospect of being replaced - then that's 18 ships of the line that the French stand to lose if hostilities with the Spanish break out. Instead of the capture of the Rosily squadron being losing 5 ships of the line (and a frigate), the French would stand to lose 18 ships of the line, and whatever frigates they had. Around 4,000 French sailors were lost, so if we're looking at 18 ships we're looking at well in excess of 15,000 sailors at the very least. Losing 15,000 people to captivity isn't something that can be brushed off lightly. Preventing the Spanish quagmire from happening would be a war-changing event, and might very well win the war then and there. If the Spanish stay as part of the Napoleonic alliance, the Portuguese occupation doesn't flare up into a rebellion or if it does without the Spanish problem the French are able to send enough troops to keep Portugal in check and the English out, then the Continental System, is immensely strengthened. English trade was decisively impacted by the Continental Blockade in the 1806/1807 period, but it recovered with trade to Portugal and the Spanish colonies. Continued Spanish hostility against English denies them this market, a French navy capable of better contesting English smuggling and hence decreasing the porous nature of the Continental System, and it might be enough to cause enough pain to bring the English to the negotiating table as the economy craters. The loss of Portugal would also mean the English would lost a vital allied nation which was important for resupply of their fleet in the theatre.
If we assume that the Spanish fleet isn't destroyed, and stays roughly around the same size, then the Franco-Spanish fleet together has around 150 ships of the line, although of course some of them are under construction. This is a large fleet in being, and as new vessels are commissioned then it raises the possibility for the French navy to be able to challenge the RN in some limited engagements or at least being able to have enough strength to be put to sea for training exercises, taking into account that only a portion of English ships can be actively deployed to blockade the French given world-wide obligations and only some of those available against the French can be used for blockading purposes to keep the French in port since only a fraction can be maintained on station.
Furthermore, if the assumption is made that the Danish fleet is spared from destruction as some above posts seem to indicate might have been the case (I don't know enough myself about whether that is a possibility but just assuming for a moment that it is), then that denies the RN 15 additional ships of the line they captured, and provides an additional neutral which would do much to help the French fleet to build up their own fleet by way of timber shipping from the Baltic on neutral Danish ships. Of course there are some advantages to the English as a continued Danish neutrality would give the British another trade outlet on the continent, but I don't know what the Danish balance of trade was with the English - most of the Baltic had a very positive balance, because they exported their timber supplies to the English, but Denmark I don't know.
If the Danes are brought onto the French side at some point (it doesn't seem unlikely given the repeated attacks the English staged on the Danes and their hostility to neutral nations which wanted to ply their commerce), then that further increases French power, with 15 extra ships of the line and a lot of competent sailors - and another area where the English have to spread themselves thin blockading.
The combination of all of this, presuming an admittedly optimal scenario, is that without Trafalgar it might be enough to either bring about an English economic collapse in the immediate years following the Continental System or in the long term places the French and Allied fleets with sufficient power to leave them well placed to start raising the costs of continued prosecution of the war to dangerous levels and possibly leaving them with the capability for pitched battles against the RN years earlier than otherwise.