strut tower bars keep(or atleast help) that particular section of the unibody from flexing... which will help corner stability and corner traction. this will also help to some extent on Drag launchs... a cross bar behind the front seats will help in launch and corner
basically what these bars do is stop the strut towers from flexing towards and away from each other... and if you have a SOLID bar (none of that swivel mount crap) they help stop the bars from flexing up and down from each other... tying into the floor helps this even more, ,and tying into the front or rear of the trunk helps in and out flex.
the stiffer and more agressive your suspension .. the more this flex is evident
personally I would worry about bracing/suspension in this order - front strut bar, rear sway bar, struts and springs, sub frame connectors, full urethane bushings, adjustable sway bar end links, mid interior brace, lower suspension braces front and rear, then Rear strut tower bar.
a side effect of this order is by the time you should be installing your rear strut bar ...you've already decided and installed your stereo/subs ... so you know where to have your strut bar made to fit