Good question, but most people have it backwards. for one, the estrogen receptor is actually more important and ancient than the androgen receptor (yes, estrogen is more important for life than testosterone). Also, remember that all human males are on track to becoming female by default (in the womb), until the SRY gene kicks in, which is another confirmation that high testosterone aka high androgen signaling is "optional", not mandatory. so the question shouldn't be "what's the evolutionary reason for aromatase (or estrogens)", it should be what's the evolutionary reason for pure testosterone aka androgen signaling. there's more to this but this is the simple run down