Left arrow is not where my fingers are, but ctrl-b is.
I would have to move my whole hand over to the arrow keys, or I can use the edge of my right hand (on a full size keyboard, not laptop) to hit ctrl, and hit "b" with my left index finger.
I use F2. Unlike the arrow keys, I don't have to move my hands and I can hit it reliably. I use it enough that I want it within reach, but not so often that I need it on the home row, and all Ctrl combinations there are already tied to emacs operations in my mind.
Same reasons you might need Ctrl-a instead of Home...
1) Some keyboards don't have it, or have it in an awkward place. Most Android on-screen keyboards don't have it (good time to plug Hacker Keyboard). The gestures on Blackberry (e.g. Android) physical keyboards act like scroll-wheel movements rather than cursor keys.
2) Some shells/systems/terminal emulators/some TERM= settings/etc. just don't handle cursor or home/end keys in the console, and instead splat out garbage like: ^[[C^[[D^[[7~^[[5~
What, and move my entire hand to the far side of the keyboard like some sort of savage? (But seriously, I actually do prefer avoiding moving my hands for ergonomic reasons.)
You should do whatever you want with your hands, but you should be aware that not moving your hands is bad for ergonomics. It promotes static posture and staying in one position for too long can cause muscle stiffness and strain. Moving around promotes blood flow and reduces fatigue. Consistent doing a small set of motions (ie never moving your hands) is what leads to RSI.
Of course, a bad setup is a bad setup, and if you're moving your hands a lot because your workspace is poorly setup, you'll also have issues.
For best results, you'll want to keep your hand movements natural and comfortable and your tools within easy reach. Take short breaks to move your hands, as well as your entire body.
What about effortless manipulation? I have CapsLock as Control and shell manipulation has become easier with readline keybinds. I can touchtype but, the extra keys are always at a different place so I don't bother learning their emplacement. So I rest my hands, but I prefer those keybinds because the already short typing bust is even shorter.
I would have to move my whole hand over to the arrow keys, or I can use the edge of my right hand (on a full size keyboard, not laptop) to hit ctrl, and hit "b" with my left index finger.