Though didn't make any real difference when troops were sent to go 'Over the Top' with bayonets fixed, and ordered not to fire till they got to the German trench, and even then, really should be using the bayonet.
Because for poorly trained troops, standing still in the middle of multiple beaten zones to engage a dug-in MMG with rifle fire is such a good idea
Make sure your magazine cutoff is engaged till the Sergeant allows rapid fire
Make sure you still have rounds in your magazine for when the Sergeant calls for rapid fire and you need to get all the rifles of the platoon engaging a priority target - when that gets called in then you need rounds on target in a big hurry and can't wait for someone to reload.
Might as well given them spears, for charging(well, actually at 'walking pace') straight into Spandau fire.
Have you seen some of these battlefields? Beaumont Hamel was something of an exception in how close the two sides were to one another, in many places it was up to half a mile. Given the weight that soldiers had to carry in order to hold a position against the inevitable imminent German counterattack (quite apart from their own basic load of ammunition, water and respirator, you need entrenching tools and your share of Bn/Coy level equipment like machine guns and their ammunition), they aren't going to be sprinting half a mile and arriving in any shape to storm a German trench. It wasn't just Spandaus on arrival either - enemy riflemen and field artillery are a major problem too, which can't simply be suppressed with rifle fire.
That lasted until well after the Somme, in 1917 when it was finally put forth that Bayonet charges should be restricted.
When the British Army had finally worked out an alternative solution enabling them to move forward further under covering fire - they still did so with fixed bayonets (even today in fact - see below), and the last bit was still done as rapidly as practicable.
The Magazine cutoff was not removed from the SMLE till 1916
That was probably done for ease of manufacture however, rather than because they considered it's removal desirable.