One of the most interesting and least appreciated facts about the course the game of football has taken over the course of the last couple of decades has been what is essentially the evolution of new full-time positions. There are more niche roles like the dimebacker, but the slot receiver and its counter, the slot defender, are now pretty much on the level of starters, which means that they require specialization to execute at a high level.
You’re not going to sign a top-tier outside cornerback in free agency, for example, and plug him into the slot. It’s entirely possible that that cornerback may even be limited to playing one side of the field.