So for the video editor, these are the obvious use cases come to mind:

User takes a video with his phone, before he shares it he wants to cut uninteresting bits out of the video and overlay a sound track.

User has a little free time while on holiday, he wants to do a simple slideshow of his photos from his trip.

User took 10-12 videos while on holiday, he wants to cut like in first use case and merge them into 1 compilation video with title intos in between and maybe mix in some tourism videos of the various places that were downloaded from the internet as the intros to each of the real video segments.