Music in iTunes is designed to be synced one way, from iTunes to the iPhone. However:
If your Mac crashed and you have all your music in your iPod, how are you going to put it back?
If your friend has some nice music in his/her iPod, how can they share it?
What about if you have 2 or more Macs with the same music library?
iRip is the solution you are looking for;
Download iRip to your Mac
Connect your iPod / iPhone via USB cable
Select the music you want to sync back to the Mac
You can choose “copy to iTunes” – Music will show up in iTunes directly
Or “copy to folders” – music will show up in specific folderOr “sync to Mac “ everything including movie and TV shows back to computer
Then make yourself a cup of coffee and wait for it to complete.