Erase and sync

Welcome to the discussions,
every time you change your computer or itunes library, all media content will be erased. Make sure all data is saved in a manual backup before you sync and your purchases were transferred.
To backup manually, disable autosync in itunes, connect your phone, right click on it in the device list and choose "backup". See what's included in the backup here: http://support.apple.com/kb/HT1766
After that, transfer your purchases the same way. Before you sync make sure to have at least one contact and one event set up on your computer, to be able to merge contacts and calendars after your phone got erased. You can restore your the rest of your data using your manual backup.
Music, if not bought in the Store, is one way only and can't be transferred back. If you back up your old itunes library you can restore it on the new PC. See how in this article: http://support.apple.com/kb/HT1382 or http://support.apple.com/kb/HT1751

I really wish someone would actually answer the question: WHY does Apple ERASE my music when I manually sync movie files? SAME library, Mac, everything. What possible reason could there be for such an idiotic function?

You can either sync or manually manage. You cannot do both. Switching between the two will erase.

Apple ( or any company ) rarely announce why and speculating is against the terms of use of this forum.