If you want to win awards you need to know what standards you brew is going to be held up against. Or if you just want to know what the heck to call that beer you’re brewing you need to know how people categorize them. The BJCP is the place to for both of these answers.
They have a list of all the beer styles and any of the attributes that the beer should have to qualify as that style. They also list cider and meads styles. I like to just read through it from time to time and get to know a style I never really thought about as a starting off point for a recipe.