That and the library access, and the fact that you don't have to fit it in around a job, obviously. Kinda goes without saying.
And you get a supervisor who might point you in the right direction and keep you on track - but I'm sure you barely need that!
Of course the equipment access and technician support goes without saying. Not like anyone else is going to be letting you use their scanning electron microscope for free.
Having peers working alongside you who share your values of intellectual curiosity, working in the same field and who are your equals academically is also of little to no importance - who needs 'em?
There's the international travel for conferences, but really that's pretty burdensome. I for one don't want to traipse through airports and mess up my routine just to have conversations with world-leading experts on my topic of interest; I'd rather be at home relaxing.
I can't imagine anyone would be drawn to the 50% female working environment, with hundreds of smart, beautiful single women aged 18-22. Who gives a shit about that? Barely even a benefit really.
Other than the intellectual curiosity, the pay, the library and journal access, the time it gives you to study, the supervision, the equipment and technicians, the peers, the international travel, and all the young women - what have the Romans ever done for us?