Why would you use this over say, Unity or Unreal? Seems more niche, less popular (so less assets/community libraries etc.) and less integrated into...everything?
Defold is optimized to produce small binaries, and features you don't use can be turned to to further decrease your bundle size.
It also focuses a lot on providing fast iteration times. You can hot-reload changed assets onto a device while the game is running. For most asset types, you'll see the change on device in less than a second. Similarly, a Build and Run cycle from scratch for a moderately complex game project such as Blossom Blast typically takes around a minute or so, and subsequent cached builds can start in a couple of seconds.
(Edit: Full disclosure, I worked on the Defold editor.)
