Why is it necessary to indicate relative to which bodies the body moves?

When a body moves, for example, along a road, then, speaking about the movement of this body, we mean that the road, houses and trees on the side of the road are motionless, and the body changes its position relative to them. But the road, houses and trees on the side of the road are not motionless, they, together with the earth’s surface, rotate around the earth’s axis, and the earth rushes around the sun at a speed of 30 km per second, etc. Hence, it should be stipulated that the case moves relative to the road surface. Or another example. A man walks along a train at a speed of 5 km / h towards an electric locomotive. The train rushes at a speed of 60 km / h. At what speed do trees outside the window sweep past a person? A person relative to the railway line moves at a speed of 60 + 5 = 65 (km / h), which means that trees will move at the same speed, but in a different direction, if the frame of reference is connected with a person.