Dropping a loaded barbell on concrete cracks the floor, damages the bar, and sends shock through your body. Proper gym flooring absorbs impact, reduces noise, protects equipment, and provides stable footing.
Horse Stall Mats: The Budget King
Worst-kept secret in home gyms: 4'x6' horse stall mats from Tractor Supply ($50 each, 3/4" thick) are the same vulcanized rubber as gym flooring at 1/3 the price. They smell for a few weeks but are functionally identical.
For general fitness: 8mm rubber mats. For weightlifting with dropped weights: 15-20mm minimum. For heavy Olympic lifting: 30mm+ or dedicated platforms.