Originally they were probably just the fatty part, with the meaty or streaky part of the bacon reserved for eating. Nowadays, we use the meaty, streaky part, and they are cut into small, usually 1 or 2 inch cubes. These are then added to the dish at the beginning cooking steps to add flavour, calories, fat, mouthfeel, and generally make a dish richer and appetizing.

 

Remember, lardons were a country, farmer, peasant, poor person's way to add meaty flavor to their basic food.

 

Other similar examples can be found in Spanish cooking (see http://foodsoftheworld.activeboards.net/forum_posts.asp?TID=472&title=trucha-a-la-navarra - trucha a la navarra ) and in the Italian method of dicing pancetta and rendering it at the beginning of many dishes - also in the use of salt pork for the beginning stages of new england clam chowder. These are jsut a few of many, many examples.