1. Bigger Screen
We'll start with the most obvious reason. The 5.5-inch display offers a close-to-phablet experience than the smaller 4.7-inch of the standard iPhone 6. The previous offers more space for multimedia consumption.
Of course, purist might say that they need some distinction between a phone and a phablet. But please, we wont be clamouring about previous iPhones having small displays if we wouldn't want them to be bigger anyways.
So there, the bigger screen, the better.
2. Customized UI just for iPhone 6 Plus
The iPhone 6 Plus will have a customized UI just for the phone's large screen. Unlike the plain iPhone 6, the 6 Plus will have an iPad-like interface. It will follow the tablet's layout in presenting apps even in landscape mode.
Aside from it having an iPad language in terms of its software, the dual-pane feature in Messages and Mails are just pleasing to the eyes.
3. More Display Pixels
I should say that this is the real deal maker. Despite the fact that iPhone 6 Plus offers a bigger screen, it also boasts more pixels with its full HD 1080p resolution. Technically, the 1920 x 1080p sums up to 401ppi density -that's more than the iPhone 6's 326ppi; and it also dubbed to have 185-percent more pixels than the iPhone 5s.
4. Optical Image Stabilization
We all know that Apple still uses the same 8-megapixel sensor that the we have been accustomed to since the iPhone 4s; but the new iPhones have remodeled True Tone Flash and the new Focus Pixels technology.
However, in terms of camera, the iPhone 6 Plus is way better than the standard iPhone 6 shooter. The Plus is the very first iPhone to have the Optical Image Stabilization which is a hardware arrangement in the camera module wherein it compesates physical shakes while taking stills and videos. The iPhone 6 only has the Digital Image Stabilization.
5. Let's talk battery
Apple made the Plus' battery big with up to a whooping 24 hours of talk time over 3G. The iPhone 5s only has 8 hours and Apple claims that the Plus' battery will outlast the iPhone 6 (14 hours) by "considerable margins."
The iPhone 6 Plus is only priced USD 100 higher than the iPhone 6 - that's just the same price difference between the iPhone 5s and 5c last year. But in my estimates, the difference compensates the additional features that the "Plus-sized" iPhone can offer in the table.