Fantasy football isn't just all about redraft leagues, where managers pick a new team every season. The fantasy world has expanded over the decades, and dynasty leagues are one of if not the most challenging and comprehensive of those expansions. In these formats, you have one main draft (a startup) where you compile a roster that you will keep year after year.
In the seasons that follow, your league will hold yearly rookie-only drafts with the goal of building not only for the present but for the future as well. Younger players are going to hold more value in dynasty leagues.