This is to support the FREE mod Block Unconfirmed Members, the mod that lets unconfirmed members log in, but whenever they attempt to go anywhere, they are always redirected to a custom page telling them they have to confirm their email first.
By letting them log in, you enable them to resend the activation mail (an option which is enable for logged in users only), but in the same time- prevent them from doing and seeing anything else in the site.
A simple mod, no core changes, easy to implement.