What are the rules for?

[Thomas Shey]

Or if you want to lift a horse, or throw it at someone, or determine if it can be supported by a bridge, or are weighing it in preparation for a transaction, or any of the other reasons why you'd want rules for the weights of objects and beings in the world. I don't care how likely I think it is to be relevant. If it exists in the game world, it has weight. And the players may choose to interact with it, in whatever manner they see fit, and they do not need any justification for any decision.

I'd suggest in a typical game, none of those three would be common; that's why I suggest a context where it might come of repeatedly and frequently.

As to the other--but of course it matters how likely it is to be relevant. Any rules set is going to stop having rules applicable to some situations somewhere. The commonest--and I'd argue best--criterion for deciding that is how likely it is to come up.