Why does he use "let" instead of "var" when building a function?

Yes, the only difference between Var (Variable) and Let (Constant) is the availability to change the value within the program. Also, I am guessing you teacher is my personal favorite, Amit! You can use the (let) for a value that you do not plan on changing the value within the program. For example: if I declare a piece of data that will be of type int that will hold my favorite number, a constant will best suite this because my favorite number will not change. He may also want to do this because values can change from time to time in more advanced programs and assigning a piece of data as a constant will ensure that the value will never change from the value that you set when you declare the constant.

//This will compile because the value of a constant can never changed, therefor you cannot assign a value to it.
let myName = "Landon"
myName = "Jacob"

//This will compile because myName is a variable, meaning the value can vary or change.
var myName = "Landon"
myName = "Jacob"

Hopefully this helps! If you have any more questions, feel free to ask!