It's a human readable backup of your private keys making you to be able to restore access to your funds if your wallet app is damaged, or you simply want to use a different wallet interface.
When it comes to using a wallet, it is always advised that we choose a very good wallet with effective security. In as much as we always encourage the hardware form of non custodial wallet, the price happens to be a big deal, though there are few other options, like software mobile wallets that can be installed on our mobile phones, and few of them have similar features like good security and Open source AFAIK. And importation using our phrase makes it easier to recover a wallet, rather than creating a fresh one.