For a very simple example of the return statement here I have a function declaration that just simply adds two numbers (or concatenates a string) and returns the product of that operation.
So the return statement is necessary when it comes to authoring any kind of function that will ultimately return a number, string or any kind of product in the form of an Object or another inner function. In this example it is just a simple single operation expression, but it could be a lengthly expression that I do not care to repeat each time I need to use it in a lengthy body of code.
For a more practical example say you want to have a function that can be used to figure out an estimate on how much ad revenue potential a certain search keyword might have.
If I know how much average monthly impression traffic a keyword has, and what the relative score and compare values of the keyword are at Google trends. Then I can use this data along with other averages for revenue per thousand and click threw rate to figure a crude estimate on keyword value.
The return keyword must be used with function expressions, and function declarations. However with arrow functions it depends on how they are authored. If brackets are not used with an arrow function then a return is implicate and the return keyword is not needed. However if brackets are used with an arrow function then the return keyword must be used just like with expressions, and declarations.