This is mainly because at times certain clients requires video signage for stadiums or large screen events and these at times requires resolution output larger than certain aspects of 4096 pixels.... Would be nice for us Media Houses if when these large scale projects comes in we don't have to be resorting to After Effects etc for larger than 4K output.
That also would be useful for 360 videos which is one thing they say it is designed for. YouTube recommends that 360 videos should use upto 8k resolution. The GoPro Fusion does a reasonable job but it's 5k has to be reduced to 4K or less for an output from Vegas 16 to YouTube. My efforts at for viewing on Samsung VR headset. Note that there are a few problems to sort out to get high resolution on the headset but when you do get it working the next problem may be travel sickness!!!!