Before bindless became a thing (and it's not yet standard with for example WebGL) then doing extra render calls just to change active textures could be an quite expensive operation. Thus a texture atlas is useful to be able to batch a lot of geometry into the same call (Also useful for 2d-like animations).

https://en.wikipedia.org/wiki/Texture_atlas

Atlases are a set of small elements grouped together in one asset. It's usually used for trees and terrain objects.