The main purpose of insoles is to absorb the impact that comes with walking every day. Our feet can take a harsh beating, especially if you pronate or walk with an unnatural gait on harsh surfaces. Insoles are designed to keep your feet in a neutral position to reduce the stress placed on your plantar fascia. When less stress is put on your plantar fascia, you will suffer fewer micro tears so your feet can heal faster and prevent Plantar Fasciitis in the future.
Insoles also provide arch support, which is important if you have Plantar Fasciitis. When you walk without enough support for your arches there is a lot of strain placed on the plantar fascia. Furthermore, if there is not enough arch support, the ligaments around your arch will not hold it in place. When this happens, your arch will collapse every time you take a step, which causes micro-tears in your plantar fascia. With insoles, your arch will be supported so the ligaments of the plantar fascia do not tear as easily.