Like other AAS, drostanolone is an agonist of the androgen receptor (AR).  It is not a substrate for 5α-reductase and is a poor substrate for 3α-hydroxysteroid dehydrogenase (3α-HSD), and therefore shows a high ratio of anabolic to androgenic activity.  As a DHT derivative, drostanolone is not a substrate for aromatase and hence cannot be aromatized into estrogenic metabolites .  While no data are available on the progestogenic activity of drostanolone, it is thought to have low or no such activity similarly to other DHT derivatives.  Since the drug is not 17α-alkylated , it is not known to cause hepatotoxicity . 
In one small scale clinical trial of depressed patients, an improvement of symptoms which included anxiety, lack of drive and desire was observed.  In patients with dysthymia , unipolar , and bipolar depression significant improvement was observed.  In this series of studies, mesterolone lead to a significant decrease in luteinizing hormone and testosterone levels.  In another study, 100 mg mesterolone cipionate was administered twice monthly.  With regards to plasma testosterone levels, there was no difference between the treated versus untreated group, and baseline luteinizing hormone levels were minimally affected. 
The second most common reason is probably to address a fear that libido might be lost without it. It is true that when added to normal androgen levels, Proviron has an androgenic effect that in many cases improves libido. However, most anabolic steroids also have this same property. In a steroid cycle, adding Proviron accomplishes nothing further. Or in the case of anabolic steroids such as nandrolone ( Deca ) which for other reasons may adversely affect libido, Proviron provides no greater help against that than do various other anabolic steroids .