The Spanish would be doing this for a handout. They wanted American cash and goods and would only do this if they could get their economy subsidized, which was still in ruins after the Civil War.
Militarily, the Spanish Ports could help the Allied Supply situation in Western Europe, and while I think that the Spanish could move up from the Pyrenees and take parts of Aquitaine from the Germans, who were hightailing it out of there anyways, and had many of their troops in SW France surrender en masse after Dragoon to small units of American forward elements (a particularly interesting story I heard was one about an OSS officer, his French contact, and 3 officers from the 36th US Infantry riding ahead and negotiating the surrender of a good part of a German Division who were infected by panic of rumors of the Russians already in Germany after communications had been cut by bombing). The Spanish air force was pitiful and would add little to the fight, but it is possible that the Allies ask for them to send some units to Italy, particularly some of the ones with Civil War experience.
Basically, the Spanish DoW would do little except moderately help the Allied supply situation and rate of advance in Southern France, which was remarkable enough as it was OTL, and possibly help in the Italian theatre with some additional battle hardened units, but this would not make the war any faster in its conclusion or do much about the post war situation.
EDIT: I completely forgot about the Spanish Blue Legion. This would obviously complicate things tremendously. While the majority of the division had returned home, the holdouts, a few thousand in number, were still on the Eastern Front. Like the French SS volunteers (who famously fought their way INTO Berlin's encirclement to make their final stand there), the remaining men were fanatical in their belief in their cause and would doubtlessly prove a huge embarrassment for Franco should at some point Spanish troops meet them in battle.