While you CAN use them as "ditty bags", I'm talking about DOODY, poo, cr**... you get my meaning.
If you like to bring Fido along, PLEASE don't let it do its business ON the trail, a few feet off trail is good, a layer of dirt on it is better. But if Fido can't wait, use a small bag to pick up the poo. And guess what, it works with people poo, too!
Or, if you just can't use the scary, stinky pit toilet in most campgrounds, use a large plastic bag. Roll down the edges a few inches, hold it... back THERE... then do YOUR business.