Two issues with an amp, power conditioner, or source component: 1) Isolation from floorborne and airborne vibrations, which can subtly muddy the sound, and 2) Draining and/or tuning the components' internally generated resonances, which can cause distortion. Isolation requires decoupling. If your rack is coupled to the floor then it is providing a path for floorborne and airborne resonant energy to enter your components, unless you have taken other steps to isolate the individual shelves and/or devices. Even if the rack also drains some of that resonant energy to the floor, if it is coupled to the floor it is a two-way street for resonance. How big an issue this is will vary from system to system, and depend in part on how resolving/revealing your system is. If your amp is well made with resonance damping materials and structures designed in, and/or there are other factors (e.g., "dirty" power) contributing noise to your system, you may not hear the impact of such resonances. Size/weight alone will not provide resonance control.
See the attached photos. My 90 lb.Pathos TT is isolated by both the corian platform and the Marigo Mystery Feet. The Marigo feet are also potent drainers of internal resonances. My Modwright Marantz player is isolated by the rack, which has decoupling feet, and by the air platform, while the Dalby Lignum Vitae footers drain and tune its internal resonances. The difference these measures make is easily audible and significant.