User researchers are people who discover and uncover new problems of the users. They also test feasibility of any design before it goes into development. They are there to make sure that the team doesn't build something based on what they felt the user needs or by thinking that the team themselves are enough as users.
If a company is hiring them, it means the company is starting to become design mature. If one is part of the product team, it is absolutely necessary to interact with user researchers and know what they do.