to let the light in?

Guest welcome but your question lacks direction... as Bamboo says all plants need light so if you have in a cold frame or a propagator then the cover/lid needs to be transparent to allow the light in

and to let you see whats happening with out constantly lifting lid letting heat out

And if a cutting then the transparent lid is to let in light which even a cutting needs, but it keeps in the moisture which the plant otherwise would lose.