A Social Security Number is required for work. It’s also required for many other reasons: getting a U.S drivers licence, bank account and, I suspect, lots of other things. It’s like the U.K National Insurance number but it’s used as a more general form of identification. Basically, it’s hard to operate as an adult human being here without one.
My visa entitles me to one of these nine-digit number, but it wasn’t as straightforward getting mine: