The gap on the spark plug needs to be just right so the spark can jump the gap correctly. If your spark plug gap is too big your engine will suffer in performance because the fuel mixture won't get burnt correctly when the spark ignites the fuel is what it does.
Your engine works by basically making explosions in the cylinder every time the fuel ignites and this is what drives the pistons making your engine run.
Having the spark plug gap too big can miss up the ignition timing and cause you to have too rich of a fuel mixture.
Make sure you set the gap correctly but check on the spark plug box as some spark plugs already have the gap set to specs.
If you need to know your vehicles gap specs it should be in your user manual or under the hood by the hood latch.