Subframes and suspension mounting points flex a lot more than you imagine. Some of the really weak ones were the EG and EK rear lower arm mounts, and the Fit/Insight/CR-Z front lower arm mounts. A tie bar will reduce the flex and help the suspension to maintain proper geometry, which will allow the car to track more accurately when cornering. The car will feel more solid and suspension will feel tighter. Modern Hondas have a lot of high-tensile steel where it counts, so this is less of a problem these days. Adding a bar where rigidity is already high will obviously make less of an improvement and just add useless weight, and many times, also underbody aerodynamic drag.