The answer to your question is that there are people who let others fly their aircraft and in some cases it is to get the hours up and therefore help with the cost of ownership. To rent the aircraft out for hire it has to be on a public transport C of A and not on a private C of A.
I hire a very nice 4-seater from a lady who charges about the same as club rates, but it is a much nicer aircraft, and you can take it away for the weekend without paying any minimum hours (so long as she doesn't want it herself of course).
I also feel very honoured to have been added to the insurance policies of a couple of friends who own aircraft. However, I think I worry more than they do about returning the aircraft in pristine condition.