What exactly does "erase and sync" mean in iTunes? This is one of the scariest, vaguest warnings I've ever seen.

In my case I don't mind if it erases all of my songs and re-syncs with my new computer, but I sure as hell don't want it erasing everything on my iPhone. Why on earth would it need to do that? Why is this error so vague?

I'm definitely not willing to just try it and see what happens.
I finally found a tool that will let me transfer songs to my iPhone without iTunes, at least from my PC: CopyTrans.