Who benefits in deals that limit the number of viewers between the league and network?
There is a potential benefit for the league if they can sell the rights to another provider in another territory and bring in an additional fee. If no one else is willing to pay the fee, then no one benefits; however, if the league gave one RSN the national rights for the same fee as they paid for the regional rights just because no one else was willing to pay for a particular game, then no one would ever be willing to pay for national rights - they would pay for regional rights and hope that no one else wanted to show the game.
Often when a game is blacked out in a particular region it is because another obscure channel in that region, regardless of whether that channel is on DirecTV or available OTA, has paid for the rights for that region. Those blackouts even apply if the channel who has paid for the rights decides to preempt the broadcast and show something else. That happens all the time with WRAL in Raleigh. They have the rights to the CBS SEC games of the week, but if Duke, UNC, or NCSU are playing, they will preempt the SEC game to show one of the local teams, but the SEC game is still not available on DirecTV. I tried to get a waiver from WRAL so I could get the SEC game on the national CBS feed on DirecTV, but, of course, I was denied. WRAL has started showing the SEC games on 5.2, but that is not in HD. It is hard being a Kentucky fan in North Carolina.