Some people worry about vibration affecting their components- either at snake oil levels, or real levels (e.g. tube microphonics would be a big one, transformer hum would be a less common one). This is why audio racks tend to be very sturdy, and use a lot of wood.
Personally I don't worry about it, it is not an issue with my gear.
How does the rack save space- you can't stack your gear without the rack? (Serious question, not sarcastic)