Batteries and Chargers are always being lost and need to be replaced. You also might want a more powerful battery for your phone or other device. Make sure your phone can handle the power of the battery you're looking at. They are not always compatible. Chargers are great when travelling or doing something like making a video, you will use more power than at other times. So, having a 2nd battery around is great. Also there are chargers like auto chargers and solar chargers that allow you to charge your device while on the go. Next time you're going to a football game or overnight trip, be prepared with extra power.
You will find all the popular batteries and chargers like the solar charger that comes with great reviews and is handy if you're outdoors alot or planning a camping trip, for example.