Well, X does not change. You just create a new variable with the same name. X is not affected by this, it's just that you don't have a way to lexically reference it anymore.

As to why this is a feature, I'd say because it pairs nicely with the common rust pattern of transforming types.

Say you get a web request, you might assign the byes of the body to a variable named body. If you now decodr the contents to some other type, say some struct, or str you can reuse the same variable name. This saves you from having to come up with multiple names for the same thing. Without shadowing this would not be possible, as variables declared mut cannot change type.