This is mentioned in generator manuals and is a safety feature that will earth the generator. An earth wire and stake are not supplied with generators that we sell and are application specific. In practice, we do not see customer's do this, especially for inverter generators. It can be something that is implemented for more specialised generators on mining work sites for example. It is not common practice to 'earth' the generator with a wire and stake. If you are in doubt, and you have a specialised generator requirement that may need 'earthing', please consult an electrician.