They are mostly used in cases where the lower jaw is smaller than the upper jaw, and there is a big distance (overjet) between the upper and lower front teeth. They usually consist of an upper and a lower plate, which aim to posture the lower jaw forward to allow it to reach its maximal growth potential.
It is important to realise, that functional appliances work by modifying the growth direction of the lower jaw, rather than growing extra bone.
The best scientific studies have shown that the best time to use these appliances is while a child is actively growing during the pubertal growth spurt – which make sense right? Since you are trying to modify growth, the best time to do it is while the child is actively going through the growth spurt. There are cases, however, where treatment may be started earlier, for example in cases where the child is being teased, or if the lower front teeth are traumatising the roof of the mouth.