1) It takes physical effort. If you are not very flexible you will find that you really have to stretch to assume the position. But, as with all stretching exercises, the more time you spend in the position the easier it becomes. Ideally if you spend enough time in the position it will eventually become a relaxed position for you. Suffice it to say that the harder it is for you to assume the position the more important it is for you to do it often and for extended periods of time.
2) It takes concentration. Until your automatic system is conditioned to snap the body into the position, you must apply a bit of brain power.
How about energy savings?
You will glide farther when you start. This will allow you to take fewer strokes. Gliding a long distance from a start takes less energy than gliding a short distance and swimming the rest of the way. And greater speed. You will move faster through the water after your start which translates to faster times.
"The more your practices resemble competitions, the less chance you will have of falling apart under pressure."