The interest is in attempting to return a patient to baseline functioning how they were before the most recent health problem.
If they have a personality disorder of which you are unaware, your efforts to return them to what you consider to be “normal” will ultimately fail.
Thus, if they are pathologically dependent or pathologically avoidant by virtue of a developmental personality disorder, and you are unaware of this, you can exhaust yourself in an attempt to restore them to a level of functioning which they have never had.
Additionally, if they have even more dysfunctional personality disorders such as paranoid, schizoid or borderline, they well have been (poorly) functioning at a subaverage level for much, if not all, of their adult lives.
Think of personality is a foundation upon which our lives are built. A personality disorder can then be considered a weak foundation for the development of adequate social and occupational functioning.
If you have a patient assessed for existence of a personality disorder, you are attempting to establish what was their functional capacity prior to the current health crisis and, thus, what is the best you can expect from care you are delivering.
It also can provide guidance as to how to enlist the participation of the patient.