Wet Cleaning

A while ago I used a laundromat. What intrigued me were the directions. They were most insistent I put the detergent in before the clothes. They practically said it was pointless to even try to wash my clother unless the detergent got in first.

Does the washing machine really care? If I put the clothes in first by mistake, should I take them out to put in the detergent? If the detergrent for whatever reason has to be at the bottom, won't gravity take care of my problem for me?