Velocity is a description of both speed and direction of motion. Velocity is a vector.
The slope of a line on a distance-time graph is speed.
Average speed is computed for the entire duration of the trip, and instantaneous speed is measured at a particular instant.
Two or more velocities add by vector addition.
It shows instantaneous speed because it doesn't show the average speed across the whole trip or show a direction. Instead, it gives the speed of a particular moment.
I could record how long it takes the car to roll down however long the ramp is.
Because, for example, if the slope were 1/2, and the distance was meters and the time was in seconds, I would be traveling 2 meters per second.
2.16 m/s
1,500 kilometers