What are the rubber circles for on the back of my pc case? Should I just leave them like that if don't have a need for them? Or are they likely to let I'm dust into the motherboard?
Edit: thanks for all the replies, so just for water cooling I have no need for.
Those are rubber grommets. They'll protect cables from wearing on metal that pass through the case.
Likely for things with hard wired controllers, like fan controllers or led lighting. You can hang the controller outside of the case in the back where nobody will see it.