Think "autograph" and images of crowds of screaming fans mobbing celebrities comes to mind, or maybe a more organized event with people lining up in droves and the celebrity sitting behind a table. Either way the end result is the same: a lot of signatures leaving with a lot of satisfied fans. And that's great for all those people who now have a souvenir to cherish, but not so much for collectors. In order for a signature to be worth a lot in terms of money it has to be fairly rare.
He may have done a lot of writing, but with only 6 of them in existence William Shakespeare's signature is one of the rarest of all and is valued somewhere around $3 million dollars. And as far as present-day celebrities go it's a harder call to make, but this past year Will Ferrell topped the list of worst celebrity signers (which would, in theory, make his John Hancock just a little more rare).