What problem does this feature solve?

Resolves the given request to a module, applies all configured loaders and calls back with the generated source, the sourceMap and the module instance (usually an instance of NormalModule). Use this function if you need to know the source code of another module to generate the result.

https://webpack.js.org/api/loaders/#thisloadmodule

What does the proposed API of configuration look like?

See: https://webpack.js.org/api/loaders/#thisloadmodule