Among US carriers, Delta was the first to introduce “basic economy” fares. This is an attempt to compete with ultra-low cost carriers like Spirit and Allegiant.
The intent is that the cheapest fares in some markets will come with even fewer benefits, like not being eligible for any sort of ticket changes (even for a fee), not allowing advance seat assignments, not allowing free upgrades, etc. The logic is that this will allow them to compete with low cost carriers on price, when they might not otherwise be able to.
While “basic economy” fares were initially rolled out in select markets where low cost carriers were dominant, they’ve since spread to many more routes.