Be it a fish only aquarium or a reef aquarium, there are going to be intakes to canister filters, protein skimmers, sumps, power heads and the like. The equipment fitted is important to the quality of the seawater and its movement.
This water quality is to enhance the life of the inmates. There is a danger that the intakes to devices such as the examples given could in fact be a threat to mobile livestock such as fish and shrimps. There is also the danger that other livestock such as corals could be damaged if they grow or are placed too close.
Taking corals first, and really considering the soft variety, many are able to grow to quite a size. If this growth brings the coral anywhere near an intake then the coral tip could be sucked in and damaged.
Fish and shrimps like to find hide holes that are secure. It has been known for them to enter an intake with a view to a secure home, only to become stuck because of the water flow and subsequently die. It has also been known for a small fish to end up in the sump, having journeyed down the feed pipe. Lucky in that case and usually no harm done. Established fish could be searching for a better home, perhaps because another fish has taken over their previous residence. Newly introduced fish have a priority to find a home and safety, searching carefully and investigating anything that looks hopeful to check if it is occupied.
Quite apart from the potential danger to the trapped fish or shrimp, there is the question of the interruption to the water flow. This could be serious in certain cases if not noticed by the aquarist for a while.
The potential problem is easily avoided by always using the intake guards that are provided with powerheads, canister filters, protein skimmers and the like. If there is any other intake, perhaps one incorporated by the aquarist in the design, it is simplicity itself to put a mesh guard over it.