You can even use the Quicktime Player to record your iPhone/iPad screen in high-quality via Lightning cable, or to simply view your iOS device on a big screen (with less lag than wireless/AirPlay.)
This can even be used to sneakily record Snapchat images & movies.
IIRC the "intended" use was to record demos of your apps and games, considering that QT Player got this feature at the same time they introduced video previews to the App Store. :)