1) Why? There are already powerbanks
- So you can restore the phone to 100% charge without having to plug it in for a couple of hours. It is annoying to use a phone while it is plugged in. Even worse if you are trying to use it while it is tethered to one of those "power banks". Carry a couple of extra batteries with you and replace them as needed, and you never have to plug in. It's convenient.
- So you can easily replace the battery when it is old and will no longer hold a charge. Batteries are cheap. New phones are expensive.
- So you can remove the battery to be sure the phone is turned OFF completely. Is the entire phone powered off, or only the screen? Did the phone freeze up with a black screen while the CPU is cpntinues to run? You can hold the power button and wait, or you could remove the voltage source and have immediate certainty. This is useful if you are paranoid about someone spying on you. It's even more useful if you have dropped your phone in water. If you remove the battery immediately and dry your phone out, it usually still works afterwards. Not possible with a permanently-attached battery.
I have taken full advantage of all these things, including dropping my phone in water and then restoring it. Removable battery is a very important feature and it is the main reason I now use an LG phone instead of Samsung or Google Nexus.