Hopping is an essential technique for riding fast on technical trails.
A good bunnyhop technique can be used to jump and control the bike in the air. It starts with pumping into the transition on the takeoff, weight back to get the front wheel up then stand up to make the back wheel follow.
Use a hop to get over big obstacles on the trail but actually smaller hops can be used a lot more often to avoid crashing in root or rock sections.
Smaller hops or unweights can used a lot when you ride technical rocky and rooty trails. You will carry more speed if you don' smash into all the edges.
Sometimes you can prehop into banks, in this case you will also get an extra pump that you will miss if you jump. You can also use a pre-hop to avoid over-jumping fade aways.