Why is it indicated with respect to which bodies the body is moving?

Mechanical movement is a change in the position of the body relative to other bodies.

The body can change its position in different ways relative to different bodies or not change its position at all.

Therefore, when moving, it is imperative to indicate the body relative to which the body is moving.

The body, relative to which the motion is considered, is called the reference body.

Example: a car moves relative to the surface of the ground, it moves away relative to some trees, approaches to other trees, and is at rest relative to the driver of this car.