I was too harried, when my classic BMW motorcycle died on the side of a busy highway, 150 miles from home, to visually test both the plugs. This simple tester would have saved me from my own laziness.
I've spent years trying to get my 1975 R75/6 ready for an SF to LA road trip. Such a ride should be nothing for BMW's legendary Type 247 boxer engine, naturally I broke down less than 1/2 way there. Running at a strong 70mph, the engine cut out. I drifted to the side and ran the standard "I own an airhead" battery of tests. I checked the float bowls and carbs to ensure fuel and air were in supply. I pulled the spark plug on the side of the bike farthest from traffic, grounded it and saw a weak spark in California's mid-day sunshine. Seeing that I had fuel and spark, I assumed my BMW was once again defying the laws of physics and I needed a tow.
Had I tested both sides, which I was reluctant to do, do to my proximity to high-speed traffic, I'd have seen the opposite plug was dead. I had spare plugs with me and, at home, when I swapped them in the bike started right up.
This tester makes testing plugs, at the wire, very easy. Simply disconnect the end cap from the spark plug and insert this wire between them. It works like a test light. Crank the bike and if the light flashes, but the cylinder doesn't fire, its your plug or fueling. If there is no light, try the cables, coils and whatever else you may have upstream (my R75/6 still runs points.)
I was lazy. I could have solved this on the road but I thought I had spark and didn't want to risk crouching mere inches from 65-90 mph traffic. It isn't like pulling spark plugs is hard, but this cable is so simple I would have had no excuse and I could have just leaned over the bike to do it.
For $5 I now have an in-line spark plug tester with me wherever I go. Also spare points and a condenser.