Sender signatures are useful for...?

So it seems that a lot of people are taking it as axiomatic
that a Sender: signature provides some utility. I can think
of a lot of utility that a From: signature provides, but I
frankly am not very sure what I'd do different if I saw a
mail message with a valid Sender signature.

Can those who think that Sender signatures are useful please
tell me what they are useful for? Preferably things that
pass the "mother" test (ie, things that an average user will
derive benefit from).

               thanks, Mike