Not seeing ads is a benefit of the service. Paying for premium is to support the video makers and, yes, the service itself.

Paying for things you use is how you signal you like the product. The insane contortions people resort to in order to justify piracy boggles my mind. You either pay by watching ads or you pay the premium price. No one is entitled to YouTube. YouTube has to be paid for in order for it to exist and the creators to make stuff. That is the reason to pay for premium.