Explicit return types have a number of advantages including easier to understand code and better type safety. It is clear from the signature what the return type of the function (if any) will be.

Invalid:

function someCalc() {
  return 2 * 2;
}
function anotherCalc() {
  return;
}

Valid:

function someCalc(): number {
  return 2 * 2;
}
function anotherCalc(): void {
  return;
}