It comes up when I'm driving someone around and they ask where my car charger is (I don't have one). Or when I'm having a party at my house and my guests asks to go into my office to plug their iPhone into my computer. You don't have to live like this anymore. There is another way.
It all started years ago when I started visiting NYC for long periods of time. Navigating the city with my phone, using it as my main camera, and also using it for entertainment while I was on the subway, ate battery life at a stunning rate. I traveled everywhere with a power cable in my pocket and I would stop constantly at coffee shops and book stores to get a little extra juice. That all changed when I finally bought the Mophie Juice Pack battery case for my phone for $130 (don't worry, you don't have to spend that much anymore).
As we use our phones more and more, especially for photos and videos, we also need to charge our phones more often. I personally haven't charged my phone once during the day in the last 5 years. My phone+battery case can last all day with me literally using the phone all day long, and almost never turning it off. Even still, the majority of my friends refuse to buy a battery case. Let's lay out all of the arguments against a battery case and allow me to crush them.
But they are so expensive!
They used to be too expensive when they were made by one company. The Mophie brand still costs $99 but I wouldn't suggest it. My Mophie cases always fell apart. The cheapo $35 version works just as well and if it does break, you can buy another and still be ahead of the Mophie.
Don't you have to remove the phone from the case to connect it to a computer?
You used to but not anymore. Even the cheapest cases allow syncing while the phone remains inside the case.
They make the phone too bulky
This is probably the best argument, but most iPhone users still use a case of some kind. Battery cases are slightly bigger than your average case but I bet you'll get used to it just like I did. When I take my phone out of my case I am always impressed by just how small it is but at the same time I find it incredibly difficult to hold without a case. It's simply too thin to easily grip. The case will make the phone easier to hold and use and if you carry it in a purse, size certainly isn't an issue. If you keep your phone in your pocket, it will add some heft but it will become normal after a few days.
Doesn't it change the phone's connection?
Yes but this is actually a good thing. By using a battery case for your iPhone you are basically guaranteeing you can charge/sync your phone in any situation. The battery case will plug into the lightning connector on the bottom of the phone but the case will sync/charge with an external USB micro cable. Almost everyone has micro USB cables in their home because it's an industry standard. If you find yourself without a cable you will now have 2 options, lightning or USB micro to charge/sync your phone.
I don't own an iPhone, I have an Android smartphone
One of the downsides to Android phones is that a new one comes out every week. 3rd party accessory companies cannot make custom products for every single Android device but they do make products for the most popular products. The Galaxy S6 has an almost identical case for just $35.
With the iPhone 6s coming out tomorrow with it's battery gobbling 4k video, having some extra power is going to be even more useful. Until these phone manufacturers stop trying to make their phones paper thin and start adding actual "all day battery life," the battery case is something I personally can't live without.