Cinda, just like all other spiders, they are carnivores and eat insects. Good insects as well as bad. The larger spiders including tarantulas, can also eat small frogs, lizards, etc; so they eat the beneficials as well. But they do keep the insect population controlled, that's probably the best 'good thing' they do. And Linda is right, tarantulas most likely want to be left alone.
For some reason, I suspect it has to do with a very mild winter and an early spring, we are having an invasion of brown recluse spiders here. Those things terrify me.
