The preamp and booster pedals have the purpose of preamplifying, and therefore improving, a signal or increasing its volume by acting on its amplitude.
It is customary to insert a boost pedal in the pedalboard when you need to use it live to increase the volume in the solo parts, or in general also for rhythm parts that need to come out more in the complete mix.
At Your Music you will find a wide range of boost and preamp pedals, such as Earthquaker Devices' Xotic EP Booster and Arrows.