used to do sth. is used to express something you usually did in the past, it is more like a habbit in the past. but now you don't do it any more. eg. I used to go to that shop. ( you often went to that shop, but now you don't ) I used to lie in in the morning.(you usually lied in in the morning. now you get up early)

Simple past is normally used to indicate a single event in the past. "Used to" is used for indicating a past habit (which doesn't happen anymore now).