Aqui,
Lucky you (silly neighbour - cold frames are essential if you are going to make best use of your greenhouse)! I presume the top is clear glass.
I use mine in Spring as the staging post from the greenhouse into the garden, but an allotment neighbour uses his (he has sevral home made) to get his courgettes and runner beans off to a good start.
In the late Autumn winter they are useful for overwintering plants that don't mind the odd gentle frost (sweet peas and hardwood cuttings for example) or, again on the allotment, for winter salads
Phil
PS a lightweight one may need anchoring if the allotment plot is exposed