I can think of two reasons:
1) to be able to see the temperature without opening the door frequently. Those fridges can sometimes fluctuate a bit and frequent opening/closings can cause the motor to run a lot while it works to maintain a set temperature. This causes constant vibration, which most people argue is bad for the wine. With a glass door, you can walk by, see that the temperature is stable, and move on. Advice: keep an independent thermometer in the unit and each time you do open it, make sure the digital temperature and the standard thermometer read the same.
2) people like their friends to see their wine when they come over!