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.

Speed-time graphs show speed on the vertical axis and time on the horizontal axis. The gradient of a speed-time graph represents acceleration because: ...

Key fact The gradient of a velocity-time graph is the acceleration of the object. The steeper the line the greater the acceleration.