This is because a convex mirror (always) forms an erect, virtual and diminished image of an object, wherever the object may be located.
Also, a convex mirror has a wider field of view, relative to a plane mirror, as it is curved outwards.
Thus, a convex mirror fitted on the sid a vehicle enables the driver to view much 1 area than that would be possible with al mirror, enabling the driver to see traffic behina him/her to facilitate safe driving.