Whats the usage need for that much ram in a phone?
Future-proofing mostly, and to make sure anything you do day to day doesn't need to be reloaded. Minimum 4GB should really be the standard on any higher end 6.0.1+ device as 3GB can cause lag and sluggishness due to regular reloading of apps if you multitask several higher RAM apps like modern games, Chrome and streaming apps throughout the day.
Moving to a Note 5 from a Note 4 is a night and day difference in overall smoothness and actual usability of the OS if you are actually utilizing the devices capabilities. Going into Winter and next year any high end device with a premium $399+ price that isn't at least 6GB is taking you for a ride to hopefully get you to overpay for their next product as soon as possible.
IE: The Note 7 should have been a 6GB device, but then there would be less of a need to get the 6GB Note 8 next year with paltry Intel levels of performance advancement YOY. "Wow, the UI so smooth now! The 6GB really brings all that bloatware into check!"
OnePlus phones are one of the underappreciated saviors of this industry trying to take the fuckery out of it. Unfortunately competition is still heavily hindered since these devices can't compete on the Verizon side of the pond.
"What?! completely open competition? Fuck that, we just want to make a ton of money, actually competing would require work, who wants to do that?"
The cell industry is vastly improved from 10-15 years ago, but it's still a massively overpriced industry that needs some real change for the betterment of everyone that is in the bottom 99% of this industry's food chain.