Spores grow into a gametophyte with both .

Spores grow into a gametophyte with both male and female organs. Because of this adaptation spores are able to clone themselves. This is also known as asexual reproduction. Asexual reproduction is an advantage to plants and spores because they do not have to rely on another plant to produce the other gamete needed to reproduce. This allows them to reproduce quickly and in large numbers.

I think it is maintaining water balance since transpiration in plants is basically when the plants roots absorb water and then release the water in the form of vapour (Don't know much about plants but I think maintaining water balance is most probably the answer)