To scare off spies who are trying to steal the secret sauce recipe.
In spite of the reviewer's opinion below, it's not unknown for the gang to run low on money and to need to take temporary jobs. See High Rise Hair Raiser, for example.
The second story by Frank Strom I'm sad to say fails on several levels. It's been established that Mystery Inc. is independently wealthy, and even Shaggy and Scooby could not possibly go through all their money on food. Thus the idea of them working in a fast food restaurant horribly reminiscent of an inferior episode of Buffy the Vampire Slayer that is still better than the short does not make sense. This is a pity because Karen Matchette does a very nice job rendering the goofy duo.