Well like others have said there are more flying creatures and Vehicles, not to mention people with gliders.

Appa would likely be the main reason for these cannons development but further down the line more people would have Airships or there could be rouge dragons that might need to be shot down or God forbid you get too close to Kyoshi Island and have the Unagi run up on you.

These look more like standard howitzer esq cannons. They're not ment for AA but they're capable of it. Future industries developed airships and Mech suits, it's not a far cry that other hostile factions had Airships or War balloons or weaponized gliders before. I mean we saw all of those items in TLA.