Right now the docs for the special value just say:

The value source='*' has a special meaning, and is used to indicate that the entire object should be passed through to the field. This can be useful for creating nested representations. (See the implementation of the PaginationSerializer class for an example.)

And as far as the PaginationSerializer, the entire implementation is just this.

What exactly does the * do and what would happen in the case of PaginationSerializer if it were omitted?