[Clewza313]

In plain English, that's "introducing you to a product you wouldn't have known about otherwise", and yes, if they convince someone to buy a $99 T-shirt, this service had value for the buyer.

The negative externalities come from the fact that very few people will buy random unbranded $99 T-shirts on a whim, meaning they need to spam a million people to find the one or two buyers in the haystack.