One reason to prune is to increase yield or lower plant profile by topping the plants grow tips. The traditional method involves removing the entire growth point but there are now other methods such as the FIM which can dramatically affect the appearance of the plant and perhaps increase yield by increased branch stucture.
From a post by Bud-E
This was originally published as TIP OF THE MONTH "Page 102" in the HighTimes Magazine July 2000 issue.
To increase yeilds and maximize space, many farmers "top" their plants-snip off the top bud. Tradionally, topping your plant causes two shoots to grow back in place on the one pruned off, thus increasing the number of top/main buds. Well thanks to a loyal reader, we've topped that.
This tip came to me by snail mail from South Carolina, With a diagram and a name for this technique. The letter says, "I discovered by accident a pruning technique that all growers should know about". As I read his letter, I remembered that the same thing had happened to me many times and I never took notice. I too occasionally got up to five or six new shoots growing from what I thought was a sloppy topping attempt.
Diagram A shows the tradional method of topping: cutting the whole node off cleanly, to induce the stalk to split into two. But diagram B indicates to make the cut below the center of the node, leaving about 10% intact. This is the key, because the vegetation left behind has cells that are rapidly dividing in many different directions. This can lead to as many as EIGHT tops from a single pruning.
The contributor did not give their name, but requests this be called the "FIM" technique. He also adds, "this pruning technique could revolutionize indoor gardening"
Additional image contributed by: bubbasix