It largely boils down to the same reason scientists classify animals into taxonomies. It helps to have a framework for classifying the groups so you can refer back to them in the future.

Going back to my example with taxonomies: Yeah, you got bit by a spider, but exactly which kind of spider bit you? What do we know about those kinds of spiders, e.g. are they known for being venomous or not, does their bite have a well-known reaction in humans, etc.