Insoles for hiking boots aren't usually intended to make ill-fitting boots kinder to feet- a much better solution is to trade in the old pair and get boots that really fit properly- but they can make hiking more comfortable even if your boots fit just fine. Added cushioning can go a long way and often, hikers with particularly narrow feet find that a replacement or extra insole will take up some extra volume and keep their feet from moving around too much.
A carefully designed footbed can help stabilise the foot and ankle and absorb shock. If you suffer from ankle, knee, or hip problems that can be exactly what you need. Hikers who face this kind of issue may finding hiking poles very good for reducing the strain on vulnerable joints too. Hiking insoles can also be handy for people with one foot slightly bigger than the other. This is is not at all unusual but it does make boot fitting difficult. Buying hiking footwear to suit the smaller foot is out of the question, so the best thing to do is fit to the bigger one and use a thicker footbed to pad out the other boot.