How did this practice ever get started? I have been gardening for a good while and I don't know how many books, TV programs, and magazine articles, I have seen that indicate put a layer of gravel in the bottom of a contain before planting. Next came packing peanuts. Don't use either.
Yes, a drain hole or holes are necessary for the water to leave the container and the roots to have some aeration. Most plants can not survive with thier roots in wet soil.
But isn't putting pea gravel or small chard of broken pots in the bottom the way it has always been done. Sadly, yes the common belief is that a layer of gravel in the bottom of pots will improve drainage and keep the soil from coming out the bottom holes. The truth is this extra layer of material hinders drainage. The number one killer of plants is root rot. Roots constantly in contact with the wettest soil will lead to root.
Here are the facts about using gravel in potted plants. Potting Mix acts like a sponge and will not release water until it is saturated. Water moving though one material into another material does happen with force. In a plant container the force is gravity. When water reaches a critical mass it will penetrate the next layer. What happens is a large amount of water is left at the point of striations or layers. The wettest part of your plant container is where the plant roots reside. This 'water logged' state leads to a poor performing or dead plant.
Other references and containter information:
Video: from Growing a Greener World
White Paper from Linda Chalker- Scott Ph.D. Extension Horticulturist, Washington State Univ.
Aggie Horticulture - Media, Repotting, & Containers