The most common reason appears to be inadequate diagnostic coverage. A substantial number of patients fail to meet the existing diagnostic criteria for one of the existing anxiety, mood, eating, or personality disorders. 

Diagnosing a patient with NOS

NOS is generally used when a clinician has determined that a mental disorder is present, but the person fails to meet the criteria for one of the existing diagnostic categories. 

University of Illinois at Urbana-Champaign

Clinical utility is the extent to which DSM assists clinical decision makers in fulfilling the various clinical functions of a psychiatric classification.

Clinical utility

Widiger, T. A., & Samuel, D. B. (2005). Diagnostic categories or dimensions? A question for the Diagnostic and statistical manual of mental disorders--. Journal of abnormal psychology, 114(4), 494. https://doi.org/10.1037/0021-843X.114.4.494

Diagnostic categories or dimension? A question for the Diagnostic and Statistical Manual of Mental Disorders -- Fifth Edition

Categorical models of classification appear to be easier to use. Dimensional models are inherently more complex because they generally provide more specific and precise information.

Ease of usage I

Dimensional models could be easier to use because it provides a more valid and internally consistent means to describe a particular patient's psychopathology. 

A dimensional model might be less cumbersome because it would not require the assessment of numerous diagnostic criteria from overlapping categories to make illusory distinctions. 

Ease of usage II 

Not otherwise specified (NOS)

There is currently no consistently applied or well-understood threshold for the presence of a categorical diagnosis. 

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