Start with the recommended gap of your plugs, then you can go wider to see if you still get spark.
The idea of the tester is:
1. You can get a good look at the spark to ensure that it is not only there, but fat and blue.
2. It provides a load to your ignition system. Using the old screwdriver or sparkplug method can be hazardess to your ignitor as a good load may not happen consistently.
3. You can open up the gap to detect coils that may fail when the firing voltage goes up. Do not get carried away with the big gaps though as you may get a mis-fire which again is a no-no. Free air firing voltage may be, say, 8000v whereas combustion chamber voltage may be, say, 14,000v. This tool enables you to duplicate that.
4. It is a hands free operation allowing one to sleep while making the test instead of having high voltage flow through your body and in turn, waking you up!
5. This tool checks everything at once...if you have a bad wire, it will show up.
The best $10 you ever spent!