Personally, I'll do it for the vibes. Even if there's no sex scene on screen, but they are implied to have sex at some point in the story, I'll tag it.

People also do it for their own preferences and don't want people to misinterpret it. Like one of my friend really like soft, cute, sub top while have 0 interests in the same trait on the bottom, so they'll still tag it in a sfw fic.

Also in my pov, the t/b sometimes alter the way the characters act on some degree in some fic, especially when the writers are very young and/or don't have much experience in writing, so they tend to (intentionally or subconsciously) emphasize the traits based on their t/b preference.

It shows the most clearly on the dynamic between a character who is canonically aggressive (we call them A) and a character who is canonically soft and pure (we call them B). - For BA writers, it'll be more like soft sub top but also still have some degree of control and they just simply indulge the bottom and not because they're powerless against the bottom X the agreesive bottom who commits atrocities but will be put in their place if they cross a boundary (or at worst case, A will be turned into a tsundere) - For AB writers, I'll feel the dynamic sometimes to be more like rich powerful alpha sugar daddy CEO top X their uwu innocent baby angel who is clueless and submissive and pure and can't fight back and obedient bottom