I'm no expert, but DD1 joined a sorority, so I have a little second-hand info.
No, you don't have to join one. If you decide to join one, you have to apply and go through a multiple interview process conducted by current members, where it is decided whether or not you'll fit in. If you make the cut, you pledge. Sometimes there's hazing involved. There's a definite hierarchy among the fraternities/ sororities. DD3 spent several days comforting a friend who was rejected by the sorority she had set her heart on.
As a frat or sorority member, you spend a lot of your time partying, as far as I can tell. You can opt to live in the frat house/ sorority house or stick with university dorms or apartments. Life in the frat/ sorority house can sometimes be a good deal more comfortable than life in the dorms, though it's sometimes more like Animal House. Dorms can be a bit rough too, tbh.
As far as I can see, the main advantage to frats/ sororities is the networking opportunity. Speaking as someone who grew up in Ireland, it all seemed appallingly cliquey and with a strong whiff of the conformity culture of the 1950s - there's a lot of the 'boys will be boys' attitude, and you rarely see a sorority girl with short hair, colourfully dyed hair, lots of tattoos, piercings, or any originality to her style. In fact, there are sororities which are notable for the number of blondes they welcome into their ranks, or young women from very affluent families.
Just anecdotally, from DD3's observation (she didn't join one but had frie ds who did), a lot of sorority women do early childhood education degrees, though there are sororities for women engineers. DD joined a sorority with the reputation of being the Jewish sorority. I don't know how she got in since she's never been Jewish, but heyho.
Later, after sorority girls have graduated, they tend to be the women running the PTO, busting a gut doing fundraising, etc.