Having worked in the test and quality engineering field for most of my career has given me one perspective on this question. My time as a game producer has given me another. And my time as a development manager for developers and testers, yet another.
If you ask a random person, the answer could be something like: “To find bugs!”
And they wouldn’t be wrong, but from my experience it is a very narrow and limiting view of why you should hire a game tester.
In the end, what we want everyone we hire to do, is to create value for the company in the best possible way they can and know how.
If you are a product/service company, like most in the gaming industry, one of the major ways to create value (but not the only) is to deliver high quality complex products/services efficiently.
And one of many ways to be part of delivering high quality complex products/services efficiently is “To find bugs!”. But there are so many other opportunities .
Janet Gregory, the author of a series of books on testing and Agile, thinks that a growth mindset will enable testers to create value in different areas, not just by finding bugs . I believe this extends to all of us. We need to think broadly with regards to game testers and what they can bring to the table, and how they can add value.
Hire game testers to use the skill set and competence  they have to create value for the company in the most efficient way they can and know how, whether that is by finding bugs, through Quality Intelligence in general , or doing something we might not have even thought about.
 Building High Quality Games
 What is the difference between testing and quality management?
 What makes a good game tester?
 QI, not QA