Some engines have a spark plug cap that uses the threaded rod, usually with a spring-clip cap.

Some 70's Mopars had a molded spark cap, but a fitting inside that slipped over the barrel adapter, and would fall off if you didn't screw it on. (Ask me how I know. )

Still other engines, like our Briggs & Stratton 5hp side valve, and a Tecumseh 2-stroke 1hp tiller, have an uninsulated "O" shaped spark end that slips over the barrel. Crude, but cheap and effective.

Older cars also had this type of spark wire end. Really, really old cars.

Spark plug manufacturers have no way of knowing which application you have, so each plug is shipped with the adapter barrel.

They've been doing this for 30+ years that I know of.

Best Regards,
Jim