The button at the bottom front of iPod/iPhone/iPad/Samsung is great UX. Why other mobile manufacturers don't offer such a button at the front as well? Most smartphones have a hardware button on the side, and some at the back(!). Most of the time the phone is on a table (or other flat surface) so wouldn't it make much more sense to place the hardware button at the front?
Well first the button on the front of the iPhone isn't the power button, they place theirs on the side as well. The reason for not putting the power button on the front is that where the users can most easily reach, so that real estate is used for functions that occur more commonly like backward navigation such as the iPhone.
Also many other phone manufacturers do have button on the front bottom panel that serve roughly the same purpose:
With regards to buttons on that back:
Therefore it is arguably just as easy to use if not easier than other button placements:
LG said the change in location, odd as it may seem, was actually much more comfortable and intuitive for users.
The real benefit then comes in the form of screen real estate. By moving all buttons off the side and face of the phone you can use that extra space to maximize screen size:
By moving the buttons off the handset's edges, Woo added, it enabled LG to design phones with ultrathin bezels. The G2's bezel, for example, measures just 2.65 mm, and the G3's is even smaller at 1.15mm. And when it comes to screen real estate, smartphone users know that bigger is indeed better.
Sure this is just one case but I think it is likely that attempts at easier finger placement is the reasoning behind most mobile device button placement.
EDIT: Also note Android OS phones have these "buttons" displayed on the screen allowing them more flexibility in hardware button location: