While you're training your future carrier air wings on land, you can also get some use out of them on regular air missions. They won't train as fast as if you just leave them on Pilot Exercise. But sometimes you need a few extra FTR or NAV or CAS to grab air superiority or harass some convoys or gain the ground support combat bonus for land troops in some secondary theater, perhaps one that's otherwise empty -- and there they are. Carrier planes don't have to fly from carriers.
They do tend to have short range, so probably not the best choice for your main production. But you also don't have to leave extras in the stockpile until a carrier needs them. Take heavy air losses in a naval battle? Swap out the entire air wing for a fully trained one from your backup land base, then put the damaged one on land duty while it reinforces and trains the newbies back up.
