What other things am I missing that would be useful and/or interesting to power when not near an electrical outlet?
Well, your phone, your tablet, your GPS, your portable speakers, your music player. You know, anything you own which can be charged via USB.
I've got 2x5000 mAh, and 2x6000 mAh batteries I keep charged around the house and when I travel. They all have 1A and 2.1A outputs, as well as built in LED flashlights. Which means I can keep most of my stuff charged until I get where I'm going. Or I can charge them where I'm sitting instead of being chained to a wall charger. (And I've got a 4-port USB wall charger as well as a 3-port USB wall charger which will do 120/240V 50/60Hz.)
They've handy and convenient, but do you actually need to ask "what things that charge from USB can I charge from USB"?
If it charges from USB, and you own it ... ta da!
Years ago I decided that except for very specific things (like my camera), I won't own anything which doesn't charge with standard USB. From there, a couple of cables, a couple of batteries, and a couple of wall chargers and you can keep stuff always charged.
Once you toss proprietary cables, or anything which can't charge by USB ... you find it's a lot easier to pack what you need.
My travel electronics bag is now capable of operating everything I need from any electrical source by reusing the same USB cables. It really cuts down on the crap and clutter.