The point of having a diagnosis for a condition is really just knowing how to treat it.
There's some benefit to self-dxing IF and ONLY IF the person doing it is looking up ways to cope or curb bad behavior and not just falling back on it as an excuse. Say you self-dx some sort of attention deficit disorder and look up alternative studying habits to pull up bad grades, or look up ways to treat a depressive disorder beyond just medication. It can be really beneficial if done in the right way and can even give a doctor or therapist something to go on when you can finally go.
But unfortunately people like to do it so they can have an excuse to fall back on when their shitty behavior rears up, or for sympathy points on the internet, or so they can screech ableism when someone expects them to be accountable for their own actions.