The altitude boost is certainly not worth the work. The velocity from the plane is small but since fuel requirements go up exponentially with necessary velocit

The altitude boost is certainly not worth the work. The velocity from the plane is small but since fuel requirements go up exponentially with necessary velocity every bit helps. More important than that is that the rocket doesn't have to punch through as dense of an atmosphere when getting up to speed. That means you don't have to reinforce the rocket as much since it's maximum dynamic pressure (maxQ) is lower giving you a better mass ratio and it means that you're not losing as much delta-v to air resistance but that's only another 100 m/s or so for a decently sized rocket like the one being launched here. But more importantly you can design your engines so that they don't have to work at higher atmospheric pressures. https://en.wikipedia.org/wiki/Rocket_engine_nozzle https://en.wikipedia.org/wiki/Rocket_engine_nozzle A Merlin engine with a bell designed for taking off from sea level has a trust at sea level of 845 kN but a thrust in Vacuum of 914 kN all while burning the same amount of fuel. And a Merlin with a nozzle sized for vacuum use can go up to 934 kN when firing in vacuum, again for the same fuel flow rate.