keeps bloat down, makes you hard, gives you woodies.
It also binds to SHBG, likely with greater affinity then testosterone. This means more test in left an unbound state.
Proviron is also known for it's anti-estrogenic property. It seems to inhibit the "aromatization" of testosterone.
In addition to making you horny, proviron is used as to increase male fertility.
Lastly, their are some theorists who believe that proviron, simply by being an androgen, can in fact help keep gains without suppressing the HTPA.