The ultra signal allows the attackers to drop the Sprinkler of Doom, which is basically a huge sprinkler placed on one of the map's high points and helps in covering turf.
As defender the reason you're protecting it from the attacking teams is because it's something that balances out their numbers disadvantage. It's worth stressing that the Sprinkler only corresponds to the attacking team that collected it, so if I as Team Rock let Team Paper collect it, that's good for us in the sense that Team Scissors will lose ground (both attackers gain more clout and benefit simply from defenders placing last in Tricolor Turf Wars), but it's also bad for us in the sense that Team Paper will now have an advantage in inking turf while we're still stuck with only two teammates and a prayer.
That's also why it spawns in twice, it allows the other attacking team a chance to claim their own to make a comeback.