What exactly does "erase and sync" mean in iTunes?

I finally found a tool that will let me transfer songs to my iPhone without iTunes, at least from my PC: CopyTrans.

Extracted from Apple's Support Page:

Clicking Erase and Sync will delete any existing content currently on your device. The content will be replaced with whatever existing content is in the current iTunes library your device is connected to. If there is no content in the iTunes library, nothing will be synced to your device, and your device will be empty.

iTunes will begin copying any purchases made with any account for which the computer is authorized.

Well, if you'd like to sync with a new computer or transfer your purchases to a computer, this is the way. And for the vague, it seems pretty clear:

Erasing and syncing replaces the contents of this iPhone with the contents of this iTunes library.

It keeps being sort of scary, though.

As far as I know and from my experience it will all delete the third party apps and music, but as for the other contents and I am pretty sure that they have links on your itunes account in iCloud like photo and contacts plus messages will not be harmed.

https://discussions.apple.com/thread/1967470?start=0&tstart=0