Many computer cases have these holes / cut-outs on the frame. What are they for?
As far as I know these serve 2 purposes:
- The small lip in the middle of each shape is usually slightly bend outwards so it touches the lid/side-cover when that is on. This provides a number of electrical contact points all around the cover with improves the "Faraday-cage" effect of the case as a whole. This in turn should help preventing the computer generating electronic noise to other equipment near it. (The 6 normal retaining catches are not enough to do this: Many covers are slightly bend/warped because they are of very thin, cheap metal and don't sit flush to the entire frame because of that.)
- The other reason, which sounds a bit dubious to me to be honest, is that the little lips help to dampen vibrations in the frame and/or cover caused by the moving parts (cooling fans, hard-drives).
Editing to add some references that support the damping idea as well. Although they are not for the chassis per se, one patent covering media drives mentions vibration damping and a second patent for a hard disk frame in fact mentions both the uses.
The "holes" aren't very important, but the little fingers inside press outward against the side, lessening vibration and noise. Another way to do it is with little insulating pads glued between the frame and the side, but that adds several construction steps and cost.
These tabs are designed to every so slightly push the side panel that you slide on the side of the case off of itself to prevent vibration noises and to hold the panel more securely in place by providing tension.