I think the "Glide" phase should be renamed to something more fitting, "Deorbit Burn" springs to mind.
I think the idea is, if you want to go somewhere else on the surface that's not all that close, you ascend to the Glide altitude, fly along at (say) 2500m/s speed, then descend once you're where you want to be.
That neatly avoids clunky and complex transitions from Supercruise to Normal Space, which would otherwise be required for covering any real distance.
This would have been a critical feature in order to manage short-term persistence for surface installation states (eg. stuff you'd blown up or affected - power stations, gun emplacements...).
The SC transition is a
full instance change every time. By jumping from planetary flight to SC, you risk
losing all the states of the things on the surface... depending on whether there is any other player still down there. It's a case of "last one out, please turn off the lights" for any instance.
By adding a midway-speed "Glide Mode", FD get planetary flight to work
with the instancing system instead of against it. While you're flying around the planet, things on the surface remain in a consistent state, just the way you left them. Because you
haven't left them... and Glide Mode is there to get you from A to B fast, without cutting over instance transitions.
Conjecture, of course. But that's kind of how I expect it to work.
Guess I'll find out in a day or two!