If you're an Android OEM, there are usually two ways to implement the navigation buttons on your phone, the older way is to make hardware buttons,usually capacitive ones under the display, another more newer way is to leave the buttons for the Android UI on the screen.
HTC chose the former way on their new One smartphone, they placed two capacitive buttons that do all the navigation, so whats wrong with that? Well firstly, they may find themselves locked out of the on-screen trend that we're seeing nowadays as more and more OEM's change to on-screen buttons. Secondly, putting hardware buttons on a device in 2013 reduces the future-proof rating of said device, How? lets say for example that Android changes to gesture-based navigation in the near future, on-screen buttoned devices will do just fine, whereas their hardware-buttoned counterparts won't make much sense and will be confusing to the user.