Sometimes I just don't need to carry a big battery with me, but I know my 1 year old iPhone battery can't be trusted. If I want to be sure I can call a ride home, after a full day out and about, I bring Anker's lipstick battery with me.
While this little guy has enough capacity to completely recharge my iPhone 6, once, I find I tend to remember it in an emergency. I'll need to call a Lyft or Uber, and see that I'm sitting at a 4% battery cliff, where my phone loves to suddenly dive for zero. Charging this once and dropping it into my jacket pocket tends to get bail me out of trouble a couple times a month.