The reason could be that Trakt integrates with media server apps like Jellyfin or Plex, or apps like Kodi, and that you can thereby bring your watch history with you across apps and you don't lose it if your server crashes, library is corrupted or something... I have never used it, but I'd imagine that'd be a reason to use it. If I knew of a libre alternative, I'd actually consider using it for Jellyfin.