Type systems don't concern themselves with what data you can put into the variable. They concern themselves with what you're allowed to do with that variable. Mathmatically, it makes no sense to take the square root of a string, or call an insert() method on an object that only runs selects, or give a background the color of laptop.

The holy grail is a program that is proven correct just by the fact that you compiled it without errors. Good, strong type systems are a major step in that direction. I'm not sure we'll ever actually reach that point, but it's good to try.