If I hand you a stick, when you expect a stick, you can handle that, right?
If I hand you a rattlesnake when you expect a stick, that's a bit harder to handle, right?
If you NULL-terminate your array, making it a string, you can then hand it to functions that expect a string, and get the expected behavior. You are handing a rattlesnake to a function that expects a stick, and acting surprised when the function does something unexpected.