On a road without dedicated bus lanes, then buses and cars are essentially fungible. Who wants to be stuck in a traffic jam inside a packed bus with standing room only?

If the bus has its own lane then it becomes effectively a different mode of transport with higher capacity and lower point-to-point time. At that point it will begin to induce its own demand. Similarly, new bike lanes and metro lines are always empty at first, then they fill up. And the resulting world is much nicer than the one where everyone was in a car.