How much they help is debatable, but a true spider won't hurt your plants.
Spider mites will spin some silk on your leaves but they won't make real large fly-catching cobwebs, so if you see something that looks like it came off of Spiderman's costume you're fine.
Spider mites are tiny (they can be seen by the naked eye, but barely) and live on the underside of your plants leaves sucking out the juices. If you have them, the plant leaves will start to get speckled.