The reason is very simple, when you send the traffic you like players to be accommodated, entertained, to get promotions, offers and to keep on playing. If they are not getting emails, special offers, have support and get VIP style communication - its a mutual lost of your traffic.
That's already a common and basic approach.
Being affiliated in a good and reputable company should always be prioritized on the table. We need to build trust in all people that's why lots of factors should be considered before grabbing the opportunity to market a certain project, sites, operators, products, etc.
Remember that we aren't just marketing them but also for ourselves as well.