The issue is with verifying 'which addresses' belong to that 'wallet' (really a keyring).

And it is not a great idea... to trust a computer screen. Of course you will need to trust a screen of course. But by storing the keys offline and air-gapping a HW wallet the safety goes up immensely.

Multi-sig just improves on that for certain situations, where you have 2 independent, separately manufactured devices sign 2 different keys. If both devices don't agree on exactly what they are signing... the transaction will not go through. This is even improved when the signing devices/keys are geographically separated.