literal types

let name: "nansen"; // literal type
name = "John" // error mark

The variable can only be "nansen".

let gender: "male" | "female" | "other"; // literal type and union type
gender = "male"; // okay
gender = "female"; // okay
gender = "man"; // don't work

gender can only be "male", "female" or "other".

literal type can be very useful when you use the union type, because now more than one value can be assigned.

you might have a variable where only certain values are expected.

a benefit of both of literal types and type alias could be keeping your code just a little drier.

dry means that don't repeat yourself.