when you count number of strings you get the real number (1,2,3,4,5...), but when you wana decide which one you wanna pull...well, zenno starts its count from 0.
That is why you need to use -1, because in this case zenno will do count to 10 like this...: 0,1,2,3,4,5,6,7,8,9 --> you see that is 10 numbers. So now if you have 10 lines and you wana call last one, you are calling line number 9. <--so i hope now you understand why -1 is needed