He doesn't realize his left brake light is out. Consequently, the on-off brake light on the right side could be misconstrued for a flashing turn signal. This could create confusion for other drivers because he really intended to continue straight ahead. Confusing signals, or lack of signals, can lead to an accident. A typical car has dozens of lights which not only illuminate the road and mark the vehicle for others to see, they also provide a means of communication among drivers.
Interior lights add to driver and passenger comfort and convenience.
On our cars as in our houses, we tend to take lights for granted until, suddenly, we discover one or more that's not working. At home this is an inconvenience, but when you're sharing the road with other drivers it's dangerous. Make it a habit to take an occasional stroll around your vehicle to check all lights. Turn on your emergency flashers. Then have someone step on the brake pedal and move the turn signal lever from left to right to be sure these lights are working.