When it comes time to spend your paper wallet, you need to load that private key into some kind of device (eg. Computer, phone, table) and sign a transaction. At this stage, you have to worry about the security of that system. Is it compromised? Does it have a hacked/buggy RNG? Will it leak your keys through a side channel? The trezor does the signing securely on the device. The keys never leave it and never need to be loaded onto another device.