A huge majority of US airports aren't slot restricted,in the sense that the limitations to airlines launching service are the demand, available gate space, and etc. Then there are a few US airports which are slot restricted,like LaGuardia, Washington Reagan, or etc. These are airports where the demand greatly outweighs the supply.
Slot restrictions are intended to ensure that there aren't too many flights arriving and departing at the same time,in hopes of easing congestion as much as possible. For slot restricted airports in the US, the FAA awards the slots largely with the intention of increasing competition between carriers, or in theory keeping fares as reasonable as possible.
That's why in the case of mergers we've seen airlines forced to give up slots,when their combined presence at an airport would believe caused competition to be limited (we saw this with the merger between American and US Airways, where they had to give up some slots at DCA).
Interestingly it was announced yesterday that this fall the FAA is easing slot restrictions at Newark Liberty Airport for the first time since 2008:The post Slot Restrictions Eased At Newark Airport appeared first on One Mile at a Time.
Source: boardingarea.com