Why do snails climb trees?

I don't really know why snails climb trees?

It's an apple tree and sometimes I've noticed them, in the autumn, nibbling the over ripe apples, the ones with coddling moth holes that go brown.

These three are dead shells stuck on by last year's slime. Perhaps they're better at climbing up trees than climbing down again?

I was messing about with the wide angle zoom, which I generally use for landscapes, trying some close up shots when I noticed these patterned snail shells in the tree. Probably not the best lens to use to capture this scene but I quite like the unusual perspective and I'm happy with the sharpness of the main central snail shell.