Heated propagators are the best way to get your seedlings growing. They provide gentle bottom heat which gives a better rate of germination for seedlings and a higher strike rate for cuttings.
The cheaper propagators provide a basic basal heat ideal for a wide range of seedlings and cuttings, whereas the more expensive models give you some control over the amount of heat provided.
They are great for the earlier parts of the year especially if you don’t want to heat your whole greenhouse but want to provide enough heat for your little seedlings.