Why should I use sugar soap when repainting walls and not just any ordinary detergent?

I'm wondering if sugar soap has some sort of abrasive in it that helps the paint stick to the old surface. I find that sugar soap is useless in removing marks from walls. Or can I just sand paper the walls with fine grade sand paper?

Answers:
There is a product called TSP (Is Feng Shwee when somone comes round and messes up your stuff?

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