What does "tradedoubler" actually do?

Every time I click on a link that opens the App store on my iPhone - for instance, in one of those "free apps" applications, it goes via Safari.

In that short moment that the Safari window floats by, I can see that it goes to a "tradedoubler" link.

I looked it up: Tradedoubler is "an international performance-based digital marketing company".

Which tells me exactly nothing at all. It doesn't explain in any case why my looking up some or other iPhone app should necessarily go via them.

Other links that Google comes up with explain technicalities about "link generation". What I don't get is why Apple would need an external party to create links to, say, their own app store?

In other words, what is this "international performance-based digital marketing company" really doing?

It's not that I'm in any way bothered by them: they don't send me bills; they don't block things. They just swivel past on that odd Safari window every time I click an app store link. It's just a bit weird.

It is as if a representative of some "international performance-based cash marketing company" would suddenly pop up every time I make a cash payment and insert themselves in the transaction that was hitherto taking place between me and, say, the supermarket cashier: a guy in a business suit would take the money from my hand and hand it to the cashier.

And everyone would just stand there and act as if it's the commonest thing in the world.

That's how it feels: some inscrutable entity has inserted itself into something that I would consider to be nothing but a link - not more complicated than the "a href" sort of thing in HTML.

I just can't help wondering about it - does anyone know if there is a good solid reason for this Tradedoubler to poke its nose in my App Store business all the time?