Even using a strong password doesn’t prevent viruses to silently sit on your computer and wait until you want to transfer coins out of your wallet. This is a vulnerable point because a virus has access to the wallet file and can read your passphrase from your keyboard.
On the contrary, TREZOR never sends private keys to the computer, because when you want to send some coins out of your wallet, TREZOR asks Bitcoin software for payment details, signs the transaction internally and then sends back just a digital signature of the transaction. There’s no point where malware on your computer could access the private keys or send away your coins without your permission.
If you keep your Bitcoins in a TREZOR, this won’t happen
Source: Official website of Trezor – https://trezor.io