Like I understand why you are being downvoted here, your comment is kind of confrontational and kind of strawmanny, but the concern I think is quite valid and deserves a real response.
My response is, mental illness is something more personal than a broken bone and at present diagnosis needs to be cruder, which does make the categories a bit more "meaningless" but I don't think it gets all the way there.
So, like, take depression for example. I am being slightly unfair but hopefully on-the-nose when I say that DSM defines depression essentially as "you are too sad, too often, and you are not in a situation like mourning a parent or spouse where that level of sadness would be expected." The point is, it's a symptomatic diagnosis.
Other symptomatic diagnoses include “migraine” or “hypertension” or “diabetes” being symptomatic is not necessarily something that excludes meaningfulness, I think we could agree? But it also means that there is a difficulty with treatment. This medicine might work for your migraine but not her migraine; whereas my hypertension is caused by not getting a good night's sleep due to mild apnea and can be cured by a CPAP machine, his hypertension is caused by the fact that he weighs 350 pounds and CPAP therapy will fail.
OK, so like diabetes there is not just one major depression, and maybe some day we will distinguish between "type 1 depression, type 2 depression, gestational depression, predepression" and have specific causes subsumed as different "types" of the symptoms. Maybe not. But the label still has some sort of meaning, just like we can have "migraine medicine" as a group of things worth trying if you have migraines, or like how we can use insulin to handle diabetes in general and so on.
But then combine this with another question which is, "for mental illness, what does cured or managed even look like?" and that's where this occupational functioning criterion starts to look quite reasonable. Because the deal is that if "depression" isn't going to single out a particular cause, the causes of your depression will still likely be around, just like "we have you using insulin" has technically fixed your diabetes (that is, the symptom -- the hypoglycemia) but the cause is still not addressed. That these illnesses take place in the mind makes them harder to quantify. So we need a qualitative criterion, a "how bad is the pain from 1 to 10?", so that we can measure if the intervention is improving things.
Asking questions about your occupational functioning is thus a reasonable qualitative scale to indicate the severity of the symptoms and the success of treatment, even though it says nothing about cause. The different "buckets" of symptoms still make sense as they suggest categories of things-going-wrong and clusters of treatments-for-those-things.