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I'm not a tech whiz so someone else can give you the tech answer.
But basically vzw locks the rom on your phone so you have to use their rom and their calling/data network. When you unlock the HSPL you are able to download roms from Sprint , T-Mobile , etc. and use their networks.
Also you or someone you know can create your own rom and put exactly the apps you want on your phone and remove the ones from the mfgr. that we call bloatware , that you have no use for. Also you don't have to wait for vzw to update you to the latest rom. If you want WM 6.5.3 on your phone and a 6.5.3 rom is out there you can flash it on your phone now . And you can change to a new rom whenever you get tired of the one you have.
You can have the latest and greatest without buying a new phone every three months. That's it in a nutshell. I am sure someone will fill the holes I missed , like you can flash a upgraded radio for better reception. And so on.