It's for people too ignorant to turn their phone before videoing.
Edit: That being said, there are RARE occasions where it's appropriate to video in portrait mode, but you will pay the price if you try to post it for playback including on those sites that feel they have to put those annoying "space filling" dimmed images of your video on either side.
To be honest, I do not like looking at vertical video on a desktop monitor (My main way of using the internet). But the few times that I did record video on my smartphone, I did it in vertical. It simply is the way orientation that smartphones are meant to be operated in, from an ergonomic point of view. And since I'm using my smartphone for the video, it sure as hell isn't amazing quality that I'm getting, so I may at least be comfortable doing it.
In the same sense, it makes sense to shoot vertical if you are certain that your content is going to be consumed on a smartphone. So adding it to the M50 seems like a good idea considering the target demographic.
To ramble on a bit: One of the times I was taking such a video with my smartphone, I was on Gran Canaria, sitting in a bus that was driving me down from the mountains. The clouds had begun to creep down the mountain, making hthe ride down the serpentines fairly impressive, so it was quite a ride for someone coming from the flat, flat region of northern Germany. So in the moment I felt the tour was worth documenting and propped up my phone vertically against the window, supporting it in my hand. Basically doing it as lazily as possible, since I was exhausted by the day of hiking. Shortly after, I heard an elderly fellow who was sitting directly behind me tell his wife how he would never even consider filming vertically, since obviously TVs aren't vertical. He wasn't to charming in expressing that, essentially going on about "the youth of today" (I'm in my mid 20s) in a cliché tone. He did all this in German, so I understood him perfectly well. I don't know if he just didn't care if he came across as rude or wasn't able to consider that I may understand him perfectly well
