If you ever come across video files that won't open in QuickTime Player, you can use the free VLC Player app to view them. This popular app has been around for a long time and can also be used to convert videos to a more standard format.
You can use QuickTime Player for very simple video editing without setting up a whole project in iMovie or Final Cut Pro. With QuickTime Player, you can combine trimming, appending more clips, and splitting clips to arrange and export edited movies.
You can record your Mac's screen using only the QuickTime Player app that comes with OS X. It is easy to record and then you get to preview the result before saving.