You can run a weed eater without a spark arrestor as the main function of a spark arrestor is to prevent sparks from leaving the exhaust and possibly causing dry grass or other dry vegetation from igniting and catching on fire.
A spark arrestor on a weed eater is a screen that is inside the weed eaters muffler which is designed to reduce the risk of starting fires, especially in dry and grassy areas.
The spark arrestor screen inside the weed eaters muffler prevents the hot sparks from getting out of the exhaust.
The hot sparks that could get out of the weed eater muffler without the spark arrestor could start a fire in dry grassy areas.
spark arrestors are just a safety feature and you can use a weed eater without the spark arrestor or with a bad spark arrestor although in some places you should have it on the weed eater muffler to prevent fires, especially when you're out trimming in dry areas.
A clogged spark arrestor on a weed eater can also cause the weed eater to run poorly or not run at all.
It's legally required that the manufacturer place spark arrestors on their weed eater mufflers and even in some other small engine mufflers as well.
The primary function of the spark arrestor is to prevent sparks from escaping the muffler and igniting dry vegetation and other flammable materials.
A clogged spark arrestor can restrict the flow of exhaust and result in loss of power, poor performance and even difficulty starting or idling of the weed eater.
You should ensure that spark arrestor is clean and not clogged and in good shape.
The spark arrestor on a weed eater is a small mesh screen which sits inside the muffler and it traps hot carbon particles and other debris from the exhaust and prevents them from being expelled as sparks.