If I am secure with my passphrase is there a need to also secure a copy of wallet.dat?

I'm a noob, just reading up on all this for the past several days.
Created a coinbase account and a bitcoin wallet on my android phone.
Sent coin to the phone.
No clue where to locate wallet.dat.
I read somewhere that your phrase IS your coin.... don't understand why I'd need the .dat file.