I used these, they are great. They are to sterilize all the parts, after you clean them. Some people sterilize all their bottles and pump parts with one of those big countertop doodads, these bags are an alternative to that (at least for me they were). I found them to be great when we traveled too, and I was washing things with the hotel washcloth or paper towels - after steaming, I felt confident that my parts were really clean.
I would give everything a pretty good rinse and throw it in. I did find that sometimes the bottles still seemed to have a film, so I would actually give them a quick swirl and rinse with the bottle brush before I steamed them.
When I was at home (and had the time) I would put all my parts in the dishwasher. I mainly used the bags at night when I got home from work, so my parts would be ready to throw in the bag for the next day. Also, while at work, I would use the wipes, instead of having to wash my parts in the kitchen sink - and steam them there, and have to deal with strange looks. Worked well for me!
Hope that helps!