You must fold your tray table up during takeoff and landing, but it has absolutely nothing to do with turbulence.

Why the tray table has to be stowed

A lowered tray table puts a hard surface directly in front of a seated passenger. If an aircraft stops abruptly or people need to leave quickly, that surface can get in the way: a passenger may have to push past it, or could be thrown against it. Folding it away clears the space in front of the seat and helps keep the cabin ready for an evacuation.

That is why crews check tray tables along with other cabin items before takeoff and landing. These are phases when the aircraft is close to the ground, so there may be little time to respond if something goes wrong.

Turbulence is a different concern

Turbulence is sudden movement of the aircraft while it is in flight. A stowed table is not the main protection against it; following the seat-belt sign and keeping your belt fastened are the important steps. The takeoff-and-landing check is about keeping the cabin clear and reducing potential obstacles if passengers or crew need to move quickly.

The practical result is simple: a tray table must be out of the way before the aircraft reaches the phases when a fast exit could matter most.