Baking soda in the washer

Why did I not hear of putting baking soda in with my laundry until a few weeks ago? I was visiting a sister, and I noticed that she had no laundry detergent. There was just a box of baking soda and a small bottle of something pleasant smelling sitting by her clothes washer. I used some of each, hoping they were what I was supposed to use to clean my clothes, and my clothes were surprisingly clean and soft afterward.

Now I use baking soda all the time in our wash. I do add a little regular laundry detergent because it's hard to shake my conviction that one should use soap to wash clothes. It's probably a bit more expensive than using laundry detergent alone, but why pay less only to have clothes that don't get as clean?