What is the use of a d-t graph and a v-t graph ?
They represent the kinematic information of an object in motion, graphically.
If d vs t graph can show at times the object was moving faster, what's the time when it moved with fastest of the speed and so on. We can calculate the instantaneous velocity by tangent at any point of t, to the curve, and hence we can construct v vs t graph.
Similarly v vs t graph shows how much acceleration is there, the amount of accleration/deceleration can be found from the slope at various points on the v vs t graph, and we can plot a vs t.
Also net area gives the displacement under the curve from given time limits while the total area gives the distance travelled by the object.