Purpose of Siu Nim Tao

The purpose of the form is not to teach you the movements of Wing Chun. This is the beginner stage only. You need to use the 'big idea' to copy the intent of the movements and the structure however this is a simple and quick process.

The main purpose of the form is to distract you. To make you go back to using your big idea and give up your little idea. To make you think 'is this right?' And tense up. It is hard enough to keep from thinking when standing. Hard enough to maintain the 'little idea' state whilst not moving because you should feel nothing and it is natural to want to and try to feel something.

So one must not just to know the movements one must complete the movements with your little idea, completely relaxed and not weak. You must imbue each motion with the full force of your minds power.

It is almost pointless repeatedly practicing the moves of the form unless you are using the little idea for each one. It is possible that you may be able to distinguish some difference in your ability to use the little idea to complete the movements when directly challenged however I feel that it is more likely that you are simply removing one of the thoughts that we all get hung up on. "Does this look right?" "Am I moving the right way?" And it only helps to remove this by giving you confidence. You have done the move so many times that you don't care about the external structure and shape any more.

To summarise:
The main purpose is to distract you from maintaining your little idea. To try and trick you into bringing back your big idea and react to the movement. Your purpose when doing the form is to maintain your basic!
In the Three Stages I've mentioned before, this is the second stage.