If you've ever heard the term "GIGO" in programming and weren't sure what it meant, it means "garbage in, garbage out." What this means is, that if you have bad input going into your program, you'll in turn have bad output from your program.
What we learn from GIGO is, put in checks, and do that little bit extra so your program runs smoothly. If your user makes a mistake, prepare your program to compensate for it. You don't want your user bombarded by errors, it makes for a bad experience.