Why do biologists classify?
For convenient study of organisms.
Biologist classify organisms by considering similarities between them. The organisms which have closely related characteristics are placed in a separate domains by biologists. Domains are further divided into
For example: If you see a mushroom and start to assume: what characteristics it might have? You will suddenly ponder that mushroom belongs to Kingdom fungi. Hence, it must be a eukaryotes, an absorptive heterotrophs, non-motile and it's cell wall must be made up of chitin.
According to Linnaeus system of classification, Kingdom was the largest until of classification. But now as the domains have been introduced so domain occupy that position.
Hope it helps!
It is because it makes it more efficient to study organisms.
Think of it like sorting your school subjects' assignments. You want to sort the same subject's papers together, so that it is more efficient to organize things, and lets you keep track of things better.
Same goes for organisms. There are a lot of organisms in the world, and it would be very inefficient to not sort them into groups based on their characteristics, inheritance, etc.
So, by sorting, biologists are able to clearer understand organisms without being stuck in a pile of unorganized species list.