Just, just in cases in the future multiplayer was implemented in game, i think it will be useful for reduce lag.

Convert to Lower Part meaning was to merge many part into one. Like turning a SP built into .obj, but still playable. The physics (how the plane fly, drag, stall speed, etc) still same, but the damage control was simplified like ground target (AA Tank, SAM, and Truck).

"But what about costume wing and custom landing gear? i don't want that to merge with the other."

The part that connect to gizmoz such as rotator, piston, spring, etc will not connect to each other. So the main body and moving part was separate by moveable gizmoz. Weapon, non moveable gizmoz, default wing control surface and engine also not merge. You can also select which part that you won't merge.

The provit that happen i think was make a 500+ part plane into maybe just 50 or less. And build with high detail camo will not overlaping each colour.

Yeah that's it, i don't know it will change anything or not (i don't understand anything about game engine, i also never play MP), but that's my thought.