Even if the you won't find many references for grids in the User Interface (it's commands are integrated with the section ones), they play an important role in the layout builder. Thanks to grids you are able to define an internal space inside a section, which you can then move around inside the same section for example.
Also a background can be defined inside a grid element, giving the possibility to have a different background as the parent section.
Grids are also defining how the next elements in the hierarchy (Areas) are being arranged.