… sent out a promotional email for a friend’s product.

He benefits, because he gets in front of people he doesn’t already know.

I benefit, because I receive a commission on any sales made.

Readers benefit, because they learn of a product they otherwise wouldn’t know about; because he let me offer them a 10% discount; and because my endorsement gives them some comfort that the man and his product are of quality (they are).

Even if you love working alone (me too), it doesn’t mean you can’t partner selectively with friends to help each other.

Note as well that you don’t need to have your own products to make money from products. (Just make sure you are confident in whomever you are promoting.)