2FA means two factor authentication. It's a guarantee that even if someone hacks your password or steals your computer, if they don't also have physical access to the secondary device (phone, yubikey etc), they won't be able to gain access to your account.
If you put the secrets of 2FA into the same computer, you are back to 1FA.
However, you end up being less secure than not enabling 2FA in the first place. This is because when you enable 2FA, they can disable the regular checks that they have for 1FA accounts. It will also be incredibly difficult to regain access to the account if you don't have access to 2FA devices.