What are the advantages of velocity time graph?

Normally a position-time graph is actually a distance-time graph where the distance of an object is measured from a fixed point called the origin.

The slope (gradient) of the graph is the radial velocity - or the component of the velocity in the radial direction - of the object. That is, the component of the object's velocity in the direction towards or away from the origin.

Such a graph cannot be used to measure the component of the velocity at right angles to the radial direction. In particular, an object going around in a circle would appear t have no velocity since its distance from the origin remains constant.