The point was that { } are mainly used for random variables, but also for algebraic variables. Sets usually consist of several numbers.
Note that while 'sets' are designated with { } and are associated with random values, 'lists' are designated with [ ] and their values are not random. You can have lists of numbers and lists of strings. Lists are like arrays, that is, you can address specific elements of a list, for example if A = [4,5,6], then A[0] gives 4.