In racing, spark plugs are practically a disposable commodity. Use them for a while and throw them away. If the engine starts acting funny, replace the plugs just in case. It's cheap insurance and really not a bad idea at all.
But with all those sets of plugs going in and out, how often do you check the plug gap? After all, if a set of plugs are new, they should be good, right? Well, usually, but not always.
A spark plug a relatively simple piece of equipment, but it must perform its job in incredibly tough conditions--especially in a race car. As long as the plug can produce a strong enough spark to light the air/fuel combustion mixture, then we are happy, but did you realize that a difference in just a few thousandths of an inch between the spark plug's electrode and the ground strap can cause a plug to misfire? And while plug manufacturers try to produce every plug the same, doing that is just about impossible in a mass-production environment. Plus, there is no accounting how that plug is handled on all the steps along the path from the factory to your hands. That is why checking the gap between the electrode and ground strap on every spark plug you use is important. It won't exactly create new horsepower, but it will keep you from leaving some in the pits.
The critical distance between the spark plug's electrode and the ground strap (that piece of steel that curves out over the end of the plug) is called the "plug gap." The act of setting this distance correctly is commonly referred to as "gapping" your plugs. If the gap is too small, there isn't enough room for enough of the air/fuel mixture to get between the electrode and ground strap, so when the plug does fire there is a chance nothing will ignite. On the other hand, if the gap is too large the spark that does appear will be weak and may not be strong enough to ignite the charge.
When you throw in the fact that you are asking a spark plug to operate in a racing environment, that makes it even harder. More engine compression means a denser air/fuel charge that is harder to ignite. High flow intake ports and--once again--high compression means lots of swirl and movement of the charge inside the combustion chamber which makes ignition harder. And high-rpm engines allow the distributor little dwell time to build up a strong spark, which--you guessed it--makes firing the charge harder.
For most racing applications, you usually want the plug gap to be between 0.020 and 0.040 of an inch. Most engine builders seem to settle around 0.035 of an inch. Factors such as the type of ignition you run, cylinder heads, fuel and even timing can affect how much gap will work best for you. Generally, you want as much gap as you can get by with to ensure ignition each time the plug fires. For more specific recommendations, talk to your engine builder and even call the tech help line for your preferred plug manufacturer.
 All those thousands of dollars...  All those thousands of dollars of high-end engine parts depend on relatively cheap spark plugs to work. Give your plugs the best chance possible by gapping every one before installation. |  The most dependable way to...  The most dependable way to gap a set of plugs is with a dedicated gapping tool. It usually is a disk arrayed with several steps or pieces of wire in marked thicknesses. Without having to force it, find the thickest piece of wire that will fit between the spark plug electrode and the ground strap. This is your current gap. |  If your plug gap needs to...  If your plug gap needs to be adjusted, don't go the "old school" route of opening up the gap by prying on the end of the ground strap with a screw driver. You are just as likely to chip something as achieving the proper gap. Instead, use the notch in the gap tool to gently open up the gap at the ground strap. |
 Likewise, by sliding the notch...  Likewise, by sliding the notch farther back and applying pressure in the opposite direction, you can close down the gap. It's much more accurate than tapping the end of the plug on a table. |  While you are at it, take...  While you are at it, take a moment to inspect the quality of the ground strap. These are welded onto the plug, and the weld isn't always strong. If a weld seems flimsy, it's better to discard a questionable new plug than have the strap fall off inside the engine. |  Some fancy new plugs like...  Some fancy new plugs like this one from E3 use engineered ground straps to produce multiple spark pathways. You cannot adjust the gap on these plugs. |