In some cases you might need to change the gap of your new spark plugs to suit the application you are installing them in. Most spark plug manufacturers will pre-gap their spark plugs for their most popular application but considering that same spark plug might fit over a hundred different vehicles or engines, checking that it matches your vehicle's specifications is a good idea.
The correct spark plug gap will prevent pre-ignition and detonation as well as misfires. Certain modifications might require you to change the plug gap for optimal performance.
Adding forced induction or high-compression pistons to an engine will usually require the spark plug gap to be closed slightly. About .004"" for every 50 horsepower added is a good starting point.
Likewise, if you are using a high power ignition with increased spark voltage, you can open up the gap .002""-.005"" to accommodate the additional spark energy.
Note: It is not recommended to change the gap on spark plugs with multiple ground paths.