What is the advantage of using Realdebrid, while using CinemaHD or Stremio?

I have CinemaHD and was toying with getting Stremio, and was wondering what the difference between the two apps are.

And I couldn’t understand - maybe because I’m not familiar with Stremio - but what are the benefits of adding Realdebrid? is it because it makes buffering better? and I’ve read comments that it’s “legal” but don’t understand why.