There are various reasons. One is that we are approached by casinos we are not yet affilated with on a regular basis.
However, if Im happy with the casinos Im already affiliated with of a particular software type and have a good working relationship with them then I have no reason to add other affiliates casinos who offer the same casino games to our players.
I dont have to trial them to see if they convert, I dont have to intergrate them into our site etc etc
This advertsing space gives other online casinos a chance to get on our site who do not feature in our plans in the medium or possibly long term of being intergrated into the site in our normal way.
If there online casino converts as well as they say then the rate of our advertising in comparision to the exposure is certainly a very atttractive option. IMO.
We not just offering banners were offering Ad space. Ads work perfectly well for non gambling sites.