I was wondering why I should sanction a doubles event I'm running. I am under the impression that they scores don't influence rankings so I'm not sure what incentive there is.

Getting it listed on the PDGA site will typically draw bigger numbers in areas with good PDGA membership. You have a place on the web to display results. Players still earn PDGA points, the event shows up on their record and any cash won for pros gets added to their annual total. You are correct that players do not get ratings. The other item that's important in many places now is event insurance.