They want to see if you are sober. Having you get out is one of the best ways to tell your coordination. They had my husband get out because he told them he dropped his wallet down the seat and they were fearful he would pull a gun. Personally, if I didn't do anything wrong, then I don't care. As long as they are respectful.
Officers see a lot of things we will never understand. I was at the grocery store and had my nephews picking fruit for me and putting it into the plastic baggies. I shoved the extra bags in my purse, not realizing it, and juggling three kids on a shopping trip. I got pulled over that night, and when I opened the purse, the officer flipped out thinking I had drugs. It did not register in my head, so yes, I got upset. It was something I would never have thought of. He laughed when he saw "ACME Markets" stamped across it.