Velocity-time graphs show how the velocity (or speed) of a moving object changes with time. These graphs also show if the object is moving at a constant speed or accelerating, decelerating, or ...

The velocity time graphs give all three variables for motion–velocity which can be read off at any time. Area is displacement and gradient is acceleration. Next page ...

But if you calculate the acceleration without the graph, you are explicitly stating that the y-intercept is zero---which it might not be. So there are some actual reasons for making a graph.