I think it's a hold over from people using Lotus 123. Lotus users started
formulas with a plus (or some arithmetic symbol). Excel wants to start formulas
with an equal sign.
But excel is forgiving and won't care if you use =+A1 or just =A1.
But users know that =+A1 doesn't do anything and find it irritating.
So if you want to irritate other users, use =+. If you want to save typing,
then drop the +.