Why do we have an air force?

I'm not hating on the air force by any means (my grandfather was in the air force) but why do we have an air force in the first place? The Army has the airborne and their own aircraft The Navy has the SEALS and Naval Aviators, not to mention their own planes, jets, and helicopters. The Marine Corps also... show more I'm not hating on the air force by any means (my grandfather was in the air force) but why do we have an air force in the first place?

The Army has the airborne and their own aircraft
The Navy has the SEALS and Naval Aviators, not to mention their own planes, jets, and helicopters.
The Marine Corps also has Naval Aviators as well as the Marine Corps Combat Aircrew.
Even the Coast Guard has aviators!

I know I probably just offended a lot of people, but I just can't make sense of it. Any help?
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