let correctNum = false;

Yes, all if statements are boolean. In this very simple example, you don't need to have a separate variable. If you're just returning a statement based on the result of the condition, you don't need to have a separate variable. Boolean variables are used to save time when comparing against parts of objects so you don't have to type in the whole thing every time, or to limit scope of variables, or when you want to check a condition, set a flag, then do something and change the flag back. They are also very useful for indicating if a condition has been triggered by various different functions. For example, you might have a boolean variable called Success, and you check a bunch of different things, and if any of them give a bad result, you set Success to false, then at the end, you do if (Success) {}. There are lots of reasons for using them, but in this video, Guil is just demonstrating how you would use it, but it's probably overkill for this small program.