1) I keep track of the battery remaining percentage. When I get too low (say 20%) I plug the pack into my phone and put them both in my pocket.
2) It's more convenient for a numer of reasons:
a) I don't have to turn off my phone to charge up like I would with changing a battery. I keep runnign as if nothing has changed.
b) I don't have to open my phone up and expose it to the elements. I had a friend on a really filthy, muddy film set in the middle of winter try to replace his battery. He ended up dropping the back of the phone and the dead battery into the mud. Pain in the ass. All I have to do is plug in a cable.
b.5) if I have a protective case on my phone, replacing the battery becomes even more of a hassle.
c) I can keep using the battery pack for days on end without needing to charge it up. Better than having to carry around and make sure to keep charged 4 or 5 or 6 extra batteries.