I have 2 versions of Quicktime on macbook 7.66 and 10. I don't know why.

10 was installed as part of OS X with Snow Leopard and evrry new OS X version since. Like millions of people, I have a QuickTime Pro license for 7, and you can't apply that to QT10. So, the option to use QT 7 from the Utilities folder is available, if you download it (sometimes it'll be put there if you migrate from an older build of OS X too). If you don't use any of the Pro features of 7, you can trash it and use 10 exclusively.

One of the best features of QT7 on Lion is the ability to go to fullscreen on an external secondary display without blanking out/making other displays unavailable. I use QT7P as my default video application across the board on Lion for that reason.