The subframe which the sway bar mounts to can flex when subjected to the forces of a sway bar bolted to it. It's not so much of an issue that the subframe will break, just that if the swaybar is flexing the subframe instead of pushing on the lower control arms then you have less of an effect. Adding a tie bar stiffens up the subframe which provides a more solid mount for the sway bar to work off of.