Erase all data on the devices first? That can't be right can it?

I have purchased a new laptop as my old died a horrible death! Installed iTunes, authorized the computer. If i try to sync both my iPhone 4 or iPod touch, iTunes informs me that i will have to erase all data on the devices first? That can't be right can it? I have some music and apps on thses devices that are not showing up on itunes that i don't want to lose... Also how do i re-download previous purchases - i didn't have icloud installed when purchasing these items.

Any advice would be appreciated, thanks in advance.