These are used in place of the spark plug to hook up diagnostic tools (compression tester, leak-down tester etc.).
They can also be used to adapt a 14mm TDC whistle or piston stop to a Sporty engine.

• 12mm adapters are usually supplied along with compression and leak down kits. They are also available at some auto parts stores and online tool companies.
• If you, by chance, have bought a 'kit' in the past only to find your gauges no longer work properly, keep the fittings that came with it before tossing out the defective parts.
  These may prove to be invaluable when they are needed.

A few adapters including a tall one (which is great for some V-8's). Add the small 14mm-12mm adapter on the long one and it'll work for 12mm plug holes.
Spark Plug Hole Thread Adapters 3)