The answer is that mobile phones are amazingly flexible, and can film high resolution footage, which you’d want to combine with either live or recorded video from your desktop. And yes, you absolutely can combine the two. In this video, I show a clip generated by the Quik iOS mobile phone app, from GoPro.
There are a myriad of applications, but imagine you had already filmed B-roll background footage on your mobile, and want to use this as the background to you on screen recording a job advert or branding video (with your company workplace shown behind), or any number of other uses.
Here I show just how this can be done, using Manycam.