The name you use when declaring a function parameter is used to allow you manipulate the variable inside the function. It is like a placeholder (eg age). When calling your function, you pass in the real variable you want to manipulate (my_age). You can name them the same but internally, that won't change anything. Look here for more explanation if you want.

Functions are useful to allow you to have a better structured code (to group some common operations in one place instead of repeating it over and over).