You may call it basic mixups, but the vast majority of Mario players don't know their options for juggles. And depending on the situation, it's less about mixups, but rather just making good capitalizations, since some characters simply lack the options to escape good juggle traps. Every time I see a Mario user do an obvious U-air that gets airdodged, I cringe, when I know that a D-air would have reliably punished or frametrapped the airdodge (in which situation, that's when you can U-air your opponent).
And the thing is, the ACTUAL use for most projectiles is for land traps. This is where fireballs in juggle situations are extremely useful, since your opponent can't shield while in the air. One of the things Mario usually can't do when juggling normally is land a KO move (besides U-smash), but land trapping with fireballs is an option that lets you land D-smash more easily when juggling an opponent. There's also many situations where jumping to juggle an opponent isn't the best option, but rather waiting for your opponent to land is more optimal, and using fireballs as your opponent is landing is a way to optimize your land trap options.