Any boat that travels longer distances needs a method of steering to allow the crew to attend to tasks like using the head, making lunch or just coiling lines. Since I sail singlehanded a lot, this is one of my most important pieces of equipment. I could do without all the other instruments except the compass, although I like having them. I have an old Raymarine ST5000 autopilot which is still working fine after 18 years.
There are several different ways an autopilot can control the helm. The simplest and cheapest type is the Tiller Pilot, suitable for small boats with tiller steering. I have used one of these on boats up to 33 feet, and the performance was quite good. Only in a steep following sea did it have trouble keeping up. Because they are exposed to the weather and have removable connections, they suffer more from wear and tear than other types.
Next up is the Wheel Pilot drive, typically used on smaller sailboats with wheel steering. They are fairly easy to install, but you must manually engage and disengage the drive clutch. We once struck a log because it took a novice helmsman too long to disconnect the pilot.
Finally, my preference is a Below Decks drive. These may be electric linear, electric rotary or hydraulic. The drive need not be made by the same company as the control head. Some companies make only the drive unit.
Raymarine EV-200 control unit without drive:
Raymarine Type 1 Linear Drive
Raymarine EV-200 including Type 1 Drive