There must be a factual answer for this, although I’m stumped if I can think of one; why whenever there has been a downpour of rain, do loads of slugs and snails appear on all paved surfaces? Where have they been, and where are they going?
I know I’m asking this as if it happens everywhere, but does it?
I mostly see earthworms. The worms end up there because they can’t breathe underwater and so they kind of get flooded out of their little burrows. Of course, the minute the sun hits that sidewalk they’re going to be goners.
The Master (on the worm thing, anyway): http://www.straightdope.com/classics/a2_014.html
My WAG would be that the wet surfaces makes it easier for slugs and snail to move around, and stops them from drying out.
So they are exercising then? Wah hey, ya live and ya learn. 
But seriously; they’ve probably just come out of wet grass, so why would they be in danger of drying out? Where I live, sometimes the pavements are actually plagued with the things; does any other area of the UK have this problem? ( I’m in Manchester, England.)