The volume pedals can be used both to manage the volume, i.e. used as master volume, or to create ethereal sounds when combined with other types of effects such as delay and reverb.
There are two types of volume pedals, active or passive. The main difference lies in the presence of a buffer inside the active volume pedals, which is why it is necessary to supply electricity.
At Your Music you can find BOSS and Ernie Ball volume pedals.