(1) Any modern mobile phone will have a screen large enough to use more energy than available to run your phone all day on one charge. Unless you have chargers all over the place, it's super convenient to keep a spare charged battery in your pocket. I practiced this with my last three phones, except for my last, the HTC One, and the contrast was stark, inconvenient, and annoying. Oh and even if you have wall chargers all over and/or even a portable battery pack, when you get low on juice, do you want to be tethered to a battery pack or the wall or spend 30 seconds swapping the battery and stay un-tethered?
(2) Travelling. Same as above but even more useful. I just visited a friend in Seattle for a week and, though I brought a charger, it was super convenient to take a spare battery or two with me whenever we left the house, sometimes for the whole day. Camping; even more important. Also, when out and about, if you have one or two spare batteries on you, you don't have to do things like micro-manage screen brightness to make sure your phone doesn't die. Less squinting and less time wasted trying to keep your phone sipping energy so it doesn't die when you need it most.
(3) Phone longevity and resale value. On most phones, if your battery dies, you are either in for a heavy repair bill or you lose the whole phone. As I'm sure you know, lithium ion batteries degrade over time, eventually holding only a fraction of the charge when they were new.
Finally: Anker on Amazon sells the following for most phones: 2 batteries + charger for approximately $20.