why do mosses grow in moist places?

Moss and certain other families of plants such as ferns (pteridophytes) are very old in evolutionary terms, and need moist environments both to survive and to reproduce.

More 'recent' or 'modern' plants have developed waxy cuticles and different internal structures and cell walls to stop dehydration. And they have also developed pollen which allows for fertilisation by wind transfer, insects, etc.

Older families of plants are dependent on water not only because they dehydrate easily due to their structure, but because they have a complex lifecycle which involves fertilisation of female plants by male plants. This is acheived by the transfer of spermatophyte cells released by the male which swim through a film of water to the female. The female then develops of spores in the fruiting body (i.e. the sticky-up bit you sometimes see on moss) of the female moss which are released to grow into new moss plants.

So without a moist environment, the moss dehydrates and can die, (although some species can survive without water for a long time, there are even desert mosses!), but definitely need water to reproduce.

Matt