Like joswift said they are taking it for health. And basically only when they are experimenting with a nonestrogenic anabolic like Primo ran solo. Some just run a baseline 200mg of test mostly for estrogen when they do this, but now, for some reason, some forego any test at all and instead do actual supplemental estrogen to compensate. Maybe an interesting thing to try just to see what happens.
Though estrogen has muscle anabolic effects in itself. In fact even in Duchaine's time they experimented with an estrogenic product called Zeranol (Ralgro)(banned in sport). https://en.m.wikipedia.org/wiki/Zeranol
People also think estrogens are uniquely associated with water retention, forgetting all anabolic/androgenic steroids can cause water retention even if there is no estrogenic component. If I'm not mistaken estrogens affect fat distribution more, interestingly if menopausal women are given estrogen they lose bodyfat, especially belly fat.
You need some estrogen for proper GH and IGF-1 function and many other growth associated pathways.
I used to do the pellet conversions myself 20 years ago well aware of Ralgro. The thing is they add estrogen to the implants to make cattle bigger and so people won't take them. There is a ton of estrogen in those implants guys who take them with the estrogen are going to have problems. Even guys that were trying to separate out the estrogen were still getting too much.
Will you blow up increasing your estrogen? Absolutely that is why I don't take anti estrogens on a regular basis the coccoon is real. However, you are getting plenty of that just not taking anti estrogens you don't need to add additional estrogen just don't take Anti estrogens