Proviron lowers sex hormone binding globulin (SHBG). SHBG renders the majority of Testosterone in our bodies inactive, so by lowering it, it will allow more free testosterone. More free testosterone leads to a greater activation of the androgen receptor, which then leads to more protein synthesis, which will lead to more muscle growth. In simple terms, Proviron makes the Testosterone we have more effective.
There are a few other reasons why Proviron is used, but that is the main reason in my opinion. One of the other main reasons would be its anti estrogen properties. Instead of me going into detail about this, you can google your questions and find plenty of info regarding what you are looking for.