Why would anyone use mix for basic yellow cake?

It's literally the simplest kind of cake to make, and doesn't really even need sifting or cake flour, and there's none of the stuff that makes other cakes hard (separating eggs, mucking with melting chocolate* or heating the milk for cocoa powder.) The only way to screw it up from scratch is to over-beat it, and that's the exact same way you can screw up the mix.

It's about an extra 3 minutes with electric beaters for scratch over mix, plus however long it takes you to measure out the flour and baking powder.

(* although this is one thing where a microwave is truly a wonderful thing... after all the complaining about double boilers from my mom, I just melt it in a pyrex dish in the microwave, easy.)

Both keep for years if kept dry in sealed containers (and not like vaccuum, just screw-top), so that's hard to imagine. Then again, I'm definitely of the sort where "food" and "bread" are synonyms.