In buying the Battle Pass, you unlock Battle Pass challenges, which are lengthy lists of objectives that turn each round of "Fortnite" into something different.
Tired of trying (and likely failing) to reach first place? Why not tackle some challenges!
Go collect the letters that spell out "Fortnite," for instance, or loot eight different chests in Tilted Towers — whatever the objective may be, however silly.
It's a subtle layer of additional objectives, and one you could fully ignore, but why would you? It's a ton of fun, and it's a nice additional push that keeps me playing.
Even if you're not interested in tackling specific objectives, it's entirely likely you'll accidentally complete some just by playing the game. Often, challenges are as simple as, "Kill two players using shotguns." — the kind of thing you're likely to do in the course of play.
Thus: Having the Battle Pass is a worthwhile purchase even if you're not interested in tackling side objectives.