I think it depends on your individual perspectives and actions.

Some people may find it validating to know that they aren't just bonkers or broken.

Some people may worry about the stigma.

Some people may use a diagnosis as an excuse to behave poorly at all times.

Some people may use a diagnosis as motivation to improve themselves.

Some people may be angry and think that the professionals are out to get them or are just trying to make money off them.

Some people may feel relief, understanding, and/or acceptance in having a professional see things that others might not.

It really depends on the individuals!

For me, I find it validating to have people believe and identify things I've known for years but thought I was just overreacting/overthinking. I'm also terrified of the stigma so I only tell friends. I have also (even before diagnosis) used it as a means to accept myself, and work to improve/manage my symptoms.