Do you need wallet.dat file to restore wallet or just 12 world phrase?

I had a problem where my wallet would not connecet. Was after I updated windows. From jeroz_ 's info on lots of post I closed wallet, deleted everything but wallet.dat from the %app folder and then reopened wallet. ( I kept a copy of whole file about like 11gb). Worked perfectly, but took 5 hours. Is it impossible to redue the wallet without that wallet.dat file? Or could I use any new wallet with my 12 world phrase? I am newbe and trying to understand wallets better. I have read some horror stories using android and ios ravenwallets.

In short do I need to back up the wallet.dat file everytime I add coins? Or does my phrase cover that?

Thanks so much jeroz_. You saved me.