++ matches possessively. This means that it will hold on to what it finds and not backtrack if the match fails with the current amount of matching handled by ++.

Contrast this with A+, where it will attempt a match of a maximum amount of As, then if the match fails, an A will be "given up" by A+, and a match will be attempted again with one less A. In the case of a failing match, if ++ will suit your needs, ++ is often many times more efficient (faster at failing when it does fail to match).

In some cases, the difference between + and ++ is trivial. In other cases they are not logically equivalent. Such as in:

/A++A/ and /A+A/

# /A++A/ should never find a match, as A++ prevents A from being match
+ed after it.
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As a matter of good practice and efficiency, I use ++ wherever possible. I do the same with *+