I heard this idea that after WWII when the Air force was created they were basically given domain over all non carrier planes. The Army meanwhile desired more planes for close support but the Airforce was more interested in strategic bombers, so the Army greatly increased their attack helicopter investment to get around this limitation, but helicopters were not as effective as planes could have been. Is there any grain of truth to this notion?
Well, doctrinally speaking, helicopters provide a sort of close air support (CAS) that many other aircraft do not provide. Helicopters can generally get to their operation zone and stay there and bring firepower to bear without having to take time to 'make runs', that is going to, through, and back again to the target area.
This is because many aircraft cannot drop under a certain speed or they risk a variety of flight issues, not least of which is the loss of altitude. This is because many aircraft, especially military aircraft, depend on their thrust (and thus speed) to stay in the air.
Now, I'm not an engineer so if I'm getting any of this wrong, please correct me!
But helicopters offer a variety of tactical and strategic options for a military that aircraft traditionally can not and usually for a lower price tag.
For example, take off and landing in rough terrain
Transport of personnel and materiel or even vehicles and tanks directly into the field
Constant overwatch
Ability to hover
Ability to accompany ground forces for extended periods of time
No need for dedicated runway
So they provide plenty of reasons to invest time and money into building and maintaining a helicopter force.
Outside of the US military, helicopters actually comprise the majority of many nations' airforce, since they require much less infrastructure (and PARTS!) to maintain than it would be to maintain a full airwing of fighters or bombers.