From the factory the 2000-05 MR2 Spyder has 2 upstream O2 sensors, each reading two cylinders. If you install an aftermarket header or turbo system (all) that puts both sensors at the same spot in a single collector or pipe, AND you’re still using a stock 1ZZ ECU, you need to split one sensor signal to both ECU inputs to avoid problems with the individual fuel trims fighting each other, essentially one pair of cylinders going very rich and the other going very lean causing misfires.
This change is not needed in any other vehicle, is not needed if you have a 2ZZ swap, would be no benefit if you have a stock type header. Not needed if you’re using a Power FC or other standalone.