A cryostat is used in situations where rapid analysis of tissues is required. The water rich tissue is frozen on a quick freezing shelf inside the cryostat, this makes it very hard and it is then ready to be cut into thin sections in a microtome, also placed inside the cryostat chamber.
On the other hand to be cut in a simple microtome, the tissue needs to be first dehydrated and fixed in paraffin before it can be sectioned. It is a long procedure compared to quick sectioning in a cryostat.
The quality of sections cut in a cryostat is inferior compared to those cut in a microtome because dehydrated and paraffin embedded tissues give better sections but when it comes to quick sectioning, Cryostat is the choice.