Can anyone tell me why we perform uniform linear motion on a horizontal wooden board and why it is horizontal?

When a wooden block moves in a straight line at a constant speed, the horizontal pulling force on the wooden block and the friction force of the wooden board on the wooden block are a pair of balancing forces

The object is maintained under the balance of the two forces. Uniform linear motion occurs because the object has an initial velocity. When a force is applied to an object to change the object from rest to motion, this force is greater than the friction force of the wooden board on the object. When the object has a certain speed, as long as the traction force is equal to the sliding friction force on the object, it can Keep objects moving in a straight line at a constant speed