strut and tie bars reinforce the chassis at it's weakest points to increase chassis rigidity so that the suspension can do it's job more efficiently. allows ou to tune the suspension without having to worry about chassis flex becoming a factor.
the only negative part about reinforcement bars is the weight. some weigh more than others, with roll cages being the heaviest of the bunch... it's the stiffest way to brace a chassis, but is all steel since they protrude into the passenger cabin.