I've used the Kolo's as already mentioned with photo corners. I also have some 5x7's by Itoya Profolio, those have sleeves that you can slide prints in and out and are streamlined and compact to carry. Plain black plastic covers. Haven't used that size a lot but sometimes comes in handy. I have an older one that's 8x10 but can't think off hand of the brand.
It is useful if you want to photograph a person or their property. People want to know what you're doing with the photos, and just seeing some good work you've done is usually enough to get them to trust you.