Also, more generally, code names mean you don't have to figure out branding at product conception time.

If you're calling it "Walrus" internally, and assume that everything about it will be based on that name, and at the last minute, marketing decides that "FurSeal" tests better, then you're screwed.

If "Walrus" is just an internal mnemonic, then engineering and marketing can both beaver away and eventually release "Anthem". Or whatever.

The problem with that is that if someone leaked what a codename related to everyone would be exposed. So if I could tell you everyone working on Unicorn, and then someone let slip that Unicorn was code for iPhone suddenly everyone on the iPhone team would be known.