Because codifying areas of contention (and agreement!) helps us find other players with similar tastes and avoid those whose tastes are irreconcilably opposed? Barring that, to choose a game that emphasizes areas of consensus rather than one that emphasizes areas of contention.
Certainly that was the hope of early RPG theorists who started to codify this stuff, and by and large I think it's a noble (if often misunderstood, and misused) endeavor.
These categories give you metrics to measure systems, players and campaigns by when you're trying to determine if you'll enjoy them or not. This helps you to find a group, GM, game, adventure series, specific character concept - whatever - that suits you best and will make your gaming better.
If you DON'T know what you enjoy - if your gaming is unexamined - you'll not only be less likely to stumble across a game that well suits your preferences, there's at least a decent chance you could end up dragging down other players by persisting with a game or game element you don't much like.
Obviously, categorization can be (and, inevitably, IS) misused in discussion. A preference can be turned into a slur: Gamism becomes "being a power gamer;" storytelling and the author stance become "not roleplaying." In fact, "not roleplaying" is something that can be thrown from virtually any direction IN virtually any direction, because there will ALWAYS be an RPG that supports a given stance or agenda and its existence can be used to justify that stance or agenda as Real Roleplaying.
Even then, though, it's arguably better to be able to parse a post "Power gamers aren't roleplaying!!11" as "I am not a Gamist and do not understand or appreciate or facilitate the Gamist agenda at my table." The statements are essentially identical in content, but one is an attack and the other is a statement of fact.
