In some softwares, such as MetaMask, seed phrase acts similarly to a private key. By gaining access to your seedphrase, a hacker can gain access to your wallet in the same fashion as if he has access to your private key.
A recommended practice is writing down your seed phrase on a piece of paper, and then storing the paper in a secure manner. This "cold storage" method prevents theft even when your computer is hacked (hot storage refers to storing the information on a device linked to the internet, where the possibility of a hack is greater). While this may seem extreme, it is recommended.