Clearly, the role of sous chef is to overcome the implications of the phrase "too many chefs in the kitchen." He or she is responsible for overseeing and executing the fast pace kitchen work demands.
The bigger the restaurant, the more likely you are to find more than one sous chef. For example, restaurants that prepare their own desserts will often have a pastry chef. This position is essentially that of sous chef, but the pastry chef holds domain over desserts rather than standard fare.
Many restaurants are much smaller and employ only one sous chef to oversee all kitchen operations. A small restaurant may also demand even more. He or she may be charged with not only training and cultivating her staff's talents, but also the hiring and firing of kitchen help. Since he or she has the finger on the pulse of the kitchen, tapping him or her as the person who orders ingredients and food items is a good idea. Few people in any given restaurant are more aware of when the lettuce is beginning to go south or the supply of peppercorn is running low than a good sous chef.
A sous chef in charge of ordering supplies will have more of a direct role in finding the best ways to keep the menu's quality high and the costs low. It would be up to the sous chef to find vendors who can provide better ingredients for less money, not an easy task when you're also keeping a kitchen buzzing. Once that buzzing's done for the day, the sous chef is in charge of cleaning it once more (along with the rest of the kitchen staff), checking the stock to determine what orders should be placed the next day and, of course, creating the specials for the next day's menu. Tomorrow is, after all, another day, and sous chefs have little time to rest.
Why, precisely, would anyone put him or herself through these rigors on nearly a daily basis? One answer is that following a career as a sous chef, a person will have all of the experience (and references) needed to open a restaurant. The position of sous chef is somewhat like a final obstacle course before the money and glory of chef de cuisine -- the person on whose vision a restaurant is based. The other answer is that some people simply enjoy keeping fuses from reaching the powder keg.