For example, protection of the nut end of a bolt or screw with a plastic dome cap – or protection of the internal hex socket head from filling with dirt by use of a protective cap insert.
Protection from electrolytic corrosion
Both from sales partner Korrex, who also provide insulating bushes which offer protection from electrolytic corrosion between combinations of differing metals.
Perhaps more generally known are standard rubber and PVC grommets for associated sealing and protection of cables or pipes – along with plastic blanking plugs and screw caps for sealing round or square tubes and holes.
Then to tidy it all up when properly screwed together, nylon cable ties are the ultimate – and ubiquitous – fastening ancillary.