That ability to connect unmatched phases is secondary to the primary function of those interconnects: over those distances it is more efficient to transmit the power in DC instead of AC. [0]
Sometimes connecting unsynchronized grids is the primary reason. The Outaouais HVDC Intertie is only about 35km long, but it allows the import and export of electricity between Ontario and Quebec.
[0]: https://en.wikipedia.org/wiki/High-voltage_direct_current#Ad...