proscar and trt vs deca/ anavar supplementation

[mtd]

I am a little confused as to the rational of using finesteride with trt to minimize hair loss or other side effects. If the active metabolite of testosterone is DHT and finesteride blocks the 5-alphareductase enzyme limiting the conversion to DHT, what is the benefit to a higher serum level of testosterone?? In my ignorance, it just seems like blocking the conversion would have positive effects on hair growth, but limit the effect on muscle growth and libido. Also, is there any difference with deca or anavar?? is their active metabolite also DHT??