Wouldn't it be great if your ad network could see what a user looked like who bought or didn't buy from your site, and was able to go out and seek similar acting/looking customers? Or help suggest the best products to promote via their sites? Or be able to re-market to prospects who did buy and you want to upsell? Or if the user didn't buy and you want to serve a stronger offer to try to convert that user?
Some of the ad networks will also implement tags on more than just the confirmation page(s).
They might request/recommend that you tag your home page and/or various high traffic pages, so that they are able to see the patterns of where users are coming in, where they leave, and what products the are buying or surfing. This data enables the ad network's engine to learn more about your customers and serve up the proper offer to the right customer to increase the probability of success.