Upwork's initial big draw for me was its escrow service. Throughout my long and storied freelance writing career, I've had to deal with clients who tried to burn me by either taking forever to pay or by skipping out on their obligations. Upwork ended this headache. My clients nowadays are required to put into Upwork's escrow system the money they'll owe me. Unless a client has a legitimate, refund-qualifying gripe with my work, Upwork guarantees I'll receive payment (and usually within two weeks).
Of course, I've found other reasons to appreciate and rely on Upwork. The most important of these secondary reasons is Upwork gives me access to some fairly spectacular potential clients. Were it not for Upwork, finding these great prospects would be like looking for a needle in a haystack. Also, Upwork made it easy for me to land many of these potentials, nearly all of whom went on to distinguish themselves as truly wonderful clients (you know, the to-die-for kind).
Another important reason I'm an Upwork loyalist is the platform's reliability. It seems like it's never down, so there's no sitting around and cooling my jets.
If you're a freelancer and you want to be successful, Upwork is a must-have. I strongly recommend it.