There are many other times I've tried to do something on my phone, but been unable to without using a PC. Here's a big one - development. You can code for Linux, on Linux. You can code for Windows, on Windows. I've even coded for freaking TI calculators, *on* the calculator. But you can't code for Android on an Android device, nor can you code for iPhone on an iPhone.
You can code for Android on Android. AIDE is the best example, and it's been out for a while. https://play.google.com/store/apps/details?id=com.aide.ui
Optimization hinders evolution.