Launch from altitude
@Ken Hagan is right about the energy, but neglects an important fact of rocketry -
At 1 gee, your efficiency is zero - you're burning to just hover.
You want to accelerate as fast as the payload can stand to avoid wasting all of that first gee.
But you can't - even for wimpy manned payloads IIRC, because of supersonic "Max Q" loads if you
get going too fast where the air is still dense - mentioned on all SpaceX launches, they actually have to throttle back some.
Again, the only benefit you get from fuel is for (total gee -1). So you really don't want to have to throttle down for mere aerodynamic loads (and losses, you can melt nose cones at supersonic speeds).
Starting from where the air is thin saves this waste, so it's not quite as bad as Ken says.