Well, the air attack avoids ground bombs, mortars and cannons, and that's it. The air attack is still vulnerable to tesla, x-bow, archer towers, wizard towers, Air Defenses and infernos, plus the air bombs.
The only thing a ground attack avoids is Air Defenses/air bombs.
So, many of the air, and many of the ground troops, are essentially infuriating morons when it comes to what they prioritize to kill.
Some are drawn to defenses more than others, some to troops more, some to resources more, etc.
In a war, typically, you'd really rather take out troops and defenses first, then mop of things that don't shoot back.
Attacking an air defense with an airborne assault is more fraught with potential failures than a ground assault, as the thing you're guaranteed to be closest to is less likely to be able to shoot you.
If, say a wizard tower is faced with a Dragon on the right and a PEKKA on the left, it can't splash BOTH air and ground target at the same time...but it CAN splash 2 drags or 2 PEKKA at the same time.
As the hit points to damage per second ratios indicate, if a high hit point attacker can have a wee bit more time to do damage per second, because the defender's D/S is spilt, their targets might go down.

