The LG G2 features a 5.2-inch display - it’s big, actually that display is just a fraction of an inch smaller than the one on the first phablet, the Samsung Galaxy Note. Yet, the G2 is substancially narrower and more convenient for single handed use.
As size of smartphones increases, so does the difficulty of using buttons on the side, according to LG. The first smartphones were small and convenient for one-handed use averaging around 2.4 inches in width. That has grown to 2.7 inches now and the G2 is 2.79 inches.
Secondly, the back buttons are where your fingers naturally lay when you hold a smartphone. LG claims it’s simply more convenient.
Third, the back buttons double in function and for instance you can use the volume down button to open the camera and capture images. It’s extremely useful for selfies!
The fourth reason is that getting those buttons on the back actually allowed engineers to narrow down the side bezel and the screen looks almost edge to edge.
Those three are the main reasons for having the buttons on the back, but it’s up to you to decide whether that is meaningful innovation or just a gimmick, what do you think?