They let you see how one fraction compares to another for one. Say you had 2/5 and 3/7 it may be tricky to tell which is bigger, but having the tiles will let you see that 3/7 is slightly bigger. And there are many many more complicated examples. So if you know which is bigger you will know subtracting the smaller one will keep it positive, while if you subtract the larger from the smaller one it will be negative.
Also, you can then find equivalent tiles. In the example I gave you would be able to see 2/5 = 14/35 and 3/7 = 15/35. Which are like fractions you can subtract simply.
