[jwiz]

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.

[aaronbaugher]

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.