I have a number of industrial racks I use for servers and storage. They're a fairly standard design: angle iron and particle board. Interestingly, some came with no feet (which would be fine on concrete, but I have actual tile floors). Some came with only four feet. Some came with eight feet, the other four serving as caps on top of the angle-iron uprights.
These are small plastic feet or caps. As feet, they protect my floor. As caps, they improve the appearance of the racks and make it less likely that I will damage items (or myself) when I'm working near the exposed tops of the angle iron posts.