Line Service is one of the oldest ways to protect a line from chafe and wear. It was used on tall ships to protect the halyards where they would pass through shivs, where dock lines would pass through hawse pipes, and where stays would be subjected to chafe. When steel rigging first started, service was used to prevent rust and protect the cable from the elements.
Three strand line was traditionally used for everything on a boat. It was the sheets, halyards, stays, lashings, dock lines, steering cables, ect. In locations where it was going to be subjected to a lot of wear, it would be "Wormed, Parceled, and Serviced".
Worming is the process of filling in the grooves between the lays with a smaller cord. Then the line is Parceled, which involves wrapping a cloth soaked in tar over the line to hold the worming in place and smooth the outer surface of the lines. 3 lay would have 3 peaks that would chafe more and 3 valleys. Worming fills these valleys and the parceling covers them up. Worming and parceling go with the lay of the line, while service goes against the lay. It is important to parcel the line starting at the bottom of the line working your way up with each wrap overlapping the next. This will create a shingle effect, helping to keep water from entering the line when it is set up as a stay. The tar helps to seal out any voids that might be left, further preventing the ingress of water which could lead to degradation of the line.
Lastly, the line is serviced. Service is applied opposite to the lay of the line. The reason is, as the line is stretched, it will straighten out a bit; unwinding its lays just a bit. If the service were applied with the lay as worming and parceling were, the service would loosen when the line is stretched. By going against the lay, it will actually tighten even more when the line is tensioned. After the line is set up, it must be coated in slurry to help keep it waterproof. Slurry is a combination of stockholm tar, varnish, and black paint. The idea is to refresh the tar so that it keeps water out while forming an external hardened layer that won't rub off.
Hemp rope needed to stay away from water to prevent rot, whereas nylon is much more resistant to rot. Nylon rope still benefits from service to prevent chafe, but doesn't need the slurry or tar applied. This allows nylon rope to be parceled using "Friction Tape" usually marketed as Hockey Stick tape.
Galvanized wire really benefits from worm, parcel, and service. Galvanized wire is more rust resistant due to its coating, but with time, the galvanized layer will wear off and corrosion will set in. By shielding it from the elements, galvanized wire can be protected in a cocoon of tar and slurry where it will remain indefinitely. In this environment, it won't suffer oxidation because there is no air exposure and it won't corrode because there is no moisture present.
Galvanized rigging that is worm, parceled, and serviced sounds amazing! But if there is any lapse in maintenance, corrosion could set in and there would be no way of inspecting it. If you opt for this type of rigging, you must commit to regularly climbing the mast and painting slurry on all sides of each stay. My concern was if I went away for a month on vacation or if the harsh part of winter lasted for too long, my maintenance would lapse and the problems could set in. Doubt would set in and I would begin to worry about the condition of my rigging. For this reason, I went with synthetics as the maintenance schedule is much more relaxed and inspection is very simple.
Synthetic rigging can benefit from service as well. Areas that will be chafed can be wrapped to form a sacrificial coating that can be replaced when needed. Since dyneema is braided, it doesn't need to be wormed. Since it doesn't suffer from rot, it doesn't need to be parceled. Lastly, since there is no lay to the line, service can be wrapped in either direction (clockwise or anti-clockwise).
Areas that certainly need service are the section on the cap shroud that passes through the spreader tip. Areas that are highly recommended to service are the ends of the headstay where the hanks will be under the most force. These areas will wear first and service helps prolong the life of the stay.