For Titanium Backup app restoration, advanced features, choice of ROM with features or optimizations not present in stock firmwares, Xposed, adblockers on Web and YouTube, etc, etc...
You don't know why rooting? Don't do it. Or go with an iPhone... [emoji6]
I'll say this. I have had my LG G3 for over a year and a half, that is longer than I have kept any phone since I began my Android venture with the G1. I had the G3 rooted for about 3 weeks. I have always had my devices rooted. I'll say this - though the option to root and unlock the bootloader would be very nice, I have found myself very fond of the LG UI - for a manufacturer UI skin, LG's takes the cake. Far less memory hungry than sense or TouchWiz, and the phone remains very snappy.
I have no need for root with the power of these devices now, the leaps and bounds in camera software, etc.
I think the software on these phones, while skin dependent can slow it down, has moved leaps and bounds past that of 3 years ago. There are some useful aspects of it. For instance, one major reason I and most others are looking heavily at this phone is the camera - well I have news for you if you didn't know - most of that is software driven, software that is not available on an AOSP based ROM. Sure, the phone might be a hair faster, but if you can't use the beautiful 16 MB camera on the back to it's full potential (The highlight of an amazing phone) is it even worth it?
Now, the answer might be yes to you, and it might even be a yes for me when I get the phone in my hands (if, lol), but I would bet that I am not alone in my stance, and I'm sure that there are others here that used to be flashaholics that no longer have a true need for it.
Of course, if you are using a Samsung device or HTC, sure I see a need because that OS skin is horrendous. BUT, with a phone with this hardware, with a fairly light skin, and from what I can see amazing camera software/hardware... Do you need to root this thing?
I suppose you can justify root without bootloader, however again, I do not need it thus probably won't go there.
I am not bashing you, simply stating where I come from now, and how my thoughts and needs have changed on the issue of ROM flashing in the last year with my still amazing G3.
I am only now looking for a new device because I felt this year would bring amazing devices... So far, the only two really worth looking at are the Nexus 6P - which is only available in the states through Google/Huwai, and this V10. Maybe the OnePlus 2. Maybe.