That’s one reason. Another less cynical reason is to ensure professionals mean the same thing when they use specific words. Brains are complicated, of course. Making predictions about how one person will react to medication based on their experiences is very difficult. Being able to collect data and group it consistently makes this prediction task much easier. “We known that many people who answer these questions with these answers improve after taking this medicine.” That’s genuinely useful, needs the DSM or something like it, and has nothing to do with insurance.