Seriously, under what circumstances will you be away from electricity power for more than two days? And that, without considering battery pakcs, a second battery or a portable solar charger...
1) Multi-day backpacking trip. Yes, you could carry extra extra battery, solar charger, etc. But, chances are these are space and weight are at a premium and those items may not make the cut. If one even remembers to bring them along.
2) Traveling to a foreign country where the requisite power adapters have not been acquired or failed to make it into the pack. Bonus for traveling through intermediate countries that have different plugs than your source or destination.
3) Shorter but busy trips where the charging just does not happen. (After a long day, arrive at hotel, go directly to bed. Wake up in the morning. Did I charge my phone? Oops. Oh, well. Gotta run.)
In some of these cases, there is no usable service anyway so you might as well turn the phone off. But I don't always remember to do this and my experience hiking is that it is very easy for me to accidentally turn the phone on and not notice if it still in my pocket.
And, of course, you need to be prepared for battery degradation. Most phones these days do not have replaceable batteries. I always cut the manufacturer's battery life in half when deciding whether it suits my needs.