Not to discourage further answers, but I'm stealing this older post from u/caffarelli, our resident eunuch historian, on what they would recommend you read:

Okay, for the big picture on eunuchs, I think I'd direct you to read this post and then perhaps this post as a follow up. I think those two answer the core question of "but why??" pretty well. This recent answer on the Byzantines from /u/arivederlestelle may also suit your mildly tickled fancies in the zone of court eunuchs. If this is not enough content I have a semi-faithfully updated list of posts of mine I like here since reddit search doesn't work too well. But yeah, lots of history here with eunuchs. :)