Acutally, the group of words are all known. Is it not possible to try all combination of the words in order to get the right pass? Something like a bruteforce attack.
Well, when you created the wallet and were asked to write down the seed phrase in order to restore the wallet if needed, you need to worry about keyloggers stealing your seed. Same with the password you use for the wallet to encrypt your private keys. This article from Forbes discusses bitcoin stealing malware and this was two years ago.
The author recommended cold storage, specifically Trezor to keep your private keys offline and the use of a hardware wallet is more important today than two years ago. If you own more than a few bitcoin you should be using a hardware wallet or risk an unpleasant surprise one day to find your bitcoin stolen.