Reduced or no spam as it disincentivizes them to an extent, incentive for relay operators to run the relay and afford server expenses, better moderation of content with user reports and you get to build micro communities to publish and receive content from.
Discussion
does joining a paid relay do this alone or also necessary to turn off public relays?
Necessary to turn off public relays or filter global feed from only paid relays. Damus for example allows you to filter global feed based on content from only paid relays.
paid , free , private - 3 type relays are essential