Certain roads run along different areas.
For example, slow speeds are required for safety is a road is near a school or in a neighborhood, because pedestrians may be walking across the road.
On a highway, no pedestrians will be there (hopefully) so higher speeds can be safely used.
Another example of using a slow speed is on a rocky or rough road, so the vehicle does not accidentally lose control.
To summarize: different roads have different speeds depending on the use of the road, these different speeds are often enforced for safety.
I hope this helped.