Diagnosis is the identification of an illness or other problem by examining the symptoms and other factors, and giving a name to whatever the apparent problem is.
Somebody influential once said something like “Diagnostic labels are only useful in the professional group that creates them. Beyond that, they’re a hindrance”. (I paraphrasing this as I remember it).*
Diagnosis is useful because it usually suggests treatment and it provides a means of talking about a problem. Quite often it can also help the afflicted person; it can be calming and reassuring to know that one’s problem has a name (and is therefore understood and treatable).
But it’s usefulness generally stops there, in the consulting room. Once the diagnosis becomes a label it can lead to all sorts of other problems. When a label becomes part of someone’s identity it leads to sentence rather than salvation.
* I wish I could find the exact quote, but I haven’t been able to. I did find Susan Heitler’s article in Psychology Today