Here's why:
* Singular: It refers to one person.
* Common: It's a general term for a person who studies living organisms.
* Noun: It's a word that names a person, place, or thing.
You could use "biologist" to refer to a specific person, like "The biologist studied the effects of pollution on the local ecosystem," or to refer to the profession in general, like "Biologists are in high demand in the scientific community."