why do you sign your mails?

i noticed lots of users pgp-sign their mails to mailing-lists. what exactly is the reason/usecase/attackvector you defend against for that? what exactly is the reason for doing so on public mailing lists? and why does it make sense to sign irrelevant messages like "+1" or "just kidding" - assuming no stego usecase is in play. -- otr fp: https://www.ctrlc.hu/~stef/otr.txt