I know it can make the length of gameobject to one, but what is the use of it?
There are many uses. I wouldn’t worry about it if you have no need for it though. One example is it can essentially turn any length vector into a direction - if you’ve just found the vector distance between two objects, it could be any length, but what if you wanted to move an object from one object to the other at a fixed speed? You can normalize it, and then multiply by the desired speed to calculate the velocity needed. If you didn’t normalize first, the velocity would be greater the further apart the objects were.