Safe defaults matter. If you're using x to index into a array, and it's randomly initialized as +-2,000,000,000 because that's what happened to be in that RAM location when the program launched, and you use it before explicitly setting it, you're gonna have a bad time.

And if you used it with a default value of 0, you're going to end up operating on the 0th item in the array. That's probably a bug and it may even be a crasher if the array has length 0 and you end up corrupting something important, but the odds of it being disastrous are much lower.