We use ours to give summer vegetables a long head start.
That means sowing tomatoes and aubergines and peppers, chillis etc about the beginning of August and getting them to about 30cm before planting out.
make sure you get a reasonable amount of air through the place to keep moulds and mildews down, and maybe use flowers of sulphur in the seed trays to help combat damping off.
Hit your local superkarket for polystyrene grape boxes that you can use for trays. They last forever, they are impervious to water and if you line them with cardboard or newspaper it helps to hold moisture without overdoing it.