To expand on what DoG said. It's about being able to debug code too.

Again with the example of the variable that should never be -1. It's easy enough to say that, but hard to enforce as DoG said. Now, lets say your program is crashing because the value is -1:

As a global variable, somewhere in your program you are writing myGloblal = (a - b)/c into the variable. Not exactly obvious that it could actually be -1 given the right a, b, and c. Then at some other point in the program you are reading the value back and crashing because you are trying to get the -1 item out of an array or something like that. Now you have to find all the places that can write into that variable and figure out which of them is writing -1 into it.

If it was passed as a parameter to a function when the program crashed, you would be able to open the program up in your debugger and see which function passed -1 to it and where it got it -1 from. This can make it much easier to debug your programs.