Like any motor, it needs to be "exercised" to keep the (for instance) fuel lines and carburator from "varnish". And prevent the oil seals from drying out.
You never know when you might need it....like during SANDY. It powered my house (more or less) for 4 days. An extension cord nightmare...but we and it did it.
Part of "preventive maintenance." Again...some, like me, will advise to do it....other will say not to. Your genny, your motor, your money, your decision. My motorhome is NOT kept in a garage and there is no electric or water where it is stored.