It's not a gimmick, it's just better user experience. Look at it through the lens of an average user rather than a power user ("power user" in the context of someone who is fluent with technical aspects of computers, not the domain).
I'm not arguing that the app itself has better video editing UX -- it may very well be that local apps win there. But for onboarding, no installation wins.
I'm not arguing that the app itself has better video editing UX -- it may very well be that local apps win there. But for onboarding, no installation wins.