It's probably gonna be optimized out by the compiler. However, linters will mark it for you, since it suggests that you actually wanted to do something with that variable and forgot about it after declaration.

... Or it can be removed to reduce visual noise once it's not necessary anymore after refactoring.

Lets say you use a variable named abcd in your function. And a variable named abcb in a for loop inside the same function. But because reasons you mistakenly use abcd inside that loop and modify the wrong variable, so that your code sometimes doesnt work properly.

It's to prevent mistakes like that.

A similar thing is to use const when the variable is not modified.

Because it's code that literally doesn't do anything. Some languages won't even let you compile without special instructions.