The cap is likely a "home remedy", an attempt to improve the PSU ripple and the PC stability. Whether it was a real help or a placebo - who knows. With worn out PSU it might had some effect.

If the enclosure is fixed to the chassis - that could have been the compartment for storing chassis accessories - screws, fixtures, standoffs. Or maybe it's a speaker enclosure?
I have a Smilodon case which has a blue box/enclosure, storing HDD rails, screws. It slides in as a shelf under the HDD section. It's not round though 😀

That round enclosure kinda looks like an ant bait station. The pest control company for the restaurant I work for uses bait stations that looks very similar to that. I'm not saying that's what it is just it looks very similar.