advantage:   When we have more and more information, we can just enlarge the length of a string but create more and more different symbols. And it means that the information will be saved easier, more economic and more steady.

how:    In a string, numbers are  saved as same as English letters, they are both called "character". For example , number '0' in ASCII is 48(decimal). Then, a number in a string (such as "213") doesn't mean its own value (two hundred and thirteen) but means a series of characters ('2','1','3').