Same reason you would buy an i7-2600K over an i5-2500K is why you would choose triple-channel over dual-channel.
For gaming, i5-2500K is 100% sufficient, without any problems. Power users though (extreme overclockers, benchmark addicts and people focused on high-performance computing like in the video editing industry would gain from the more powerful i7-2600K with something like 12GB of DDR3 in triple channel configuration. The average user/gamer wouldn't even notice a difference in performance between dual and triple channel, so there's no need to spend extreme cash on the motherboard/RAM combo at all.
Stick with dual channel, it's fine as it is