Seed phrase can be 12, 15, 18, 21 or 24 words long.
This means if your phone gets lost or spoils, you can still recover your wallet, so as far as you have your seed phrase with you. Just think of seed phrase as the master key to all your wallet.
You can have wallet on computer or have a hardware wallet.
Seed phrase is important because it can generates the private key which can be used to recover your coins or to spend the coins.