Al told me it was about making sure the contract was honored.
If they get to a gig, and there's a nice big bowl of M&Ms with no brown ones, they know that someone read every aspect of the contract, and that they can relax, and have a good show.
If they get to a gig, and there's no bowl of M&Ms, they know they're fucked.
If there's a bowl there with M&Ms, but there's brown ones in it, they know someone read the fucking rider, but maybe not so carefully, and it's time to start looking around, and asking questions to make sure all is OK.
I know it's not as glamorous sounding as "they trash the dressing room if they don't get their favorite eccentric confectionery request", but it's actually there in the rider for what turns out to be merely an easy color-coded barometric indicator of actual rider compliance, that helps them stay on top of things while on the road.
Things ain't always what deyz seems, folks.