The other of the two final megastructures, the gateways, are built around black holes. You need to build at least two of them so they can be paired, and then fleets can travel between the two gateways. They're quite useful for allowing ships to travel from one end of a massive empire to another quickly. And this is the reason I didn't bother building them, as we didn't need this bonus as much as we needed those provided by the Stellar Nexus, and we're running short on gigamass.