Building strength seems to help with all sorts of pain issues. I once banged up my arm pretty good in a fall; what helped was going to the gym and doing curls and extensions with whatever the arm could support: five pounds, ten pounds, whatever.
Blood flow is the most likely explanation there. The simple act of activating those muscles forces blood flow, which can often be restricted in a healing area. Same reason we apply warmth to an area and encourage light massage.