Because on the areas where you are interested in getting stuff done you have a reasonable negotiating partner to help you get it done. You hammer out a deal with them and you get an incremental step but you don’t have to fight every wedge issue with crazy people who are offended when non-white people might benefit from a thing. And occasionally, you might be able to vote for one of them over an extremist from your own party, because you know you aren’t handing all the power to crazies from the other side by doing that.
Also, the party’s positions would not be a set of milquetoast half-measures. They would be mostly a different mix of principles selected from both sides of the aisle. It would be less about saying, “Hooray for 1% higher minimum wages!” and more about pairing UBI with military investment and 2nd amendment rights (or something - might be the opposite).
All the parties would be minority parties, so the one that is willing to work with either side would wield enormous power to set the agenda and define the Overton window.