What are the holes at the bottom of a dump truck vessel for?
These holes are for releasing exhaust. When we work an engine, it emits exhaust. Exhaust goes through a vessel and gets released through the holes. Why does it go through a vessel?
It is for keeping a vessel warm. Soil and mud could get frozen on a vessel while a dump truck is loading them in cold areas, which may keep them stuck on a vessel when unloaded.
By running exhaust and keeping a vessel warm, we can prevent soil and mud from being frozen. This function of smoothing unloading is called "body heating".