> I like that idea as it means yiu are more likely to host a party. It is also less expensive to do so.

Yeah, you're expected to help in some way. The idea being that a party at someone's house is likely an inconvenience to that person so if you want more of that to happen you should make sure the host feels barely any pain. Even if you don't bring anything to the party you should be helping place the table, carrying furniture, doing dishes...something.

People who don't help at all aren't well perceived and will probably not be invited again.