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.