Okay, let me say it right away: Filters are the most over-used thing ever in Photoshop.

Now, that doesn't mean you shouldn't ever use them. But if you're not sure if a filter is necessary in some particular case, you're probably better off not using it.

That said, SOME filters are insanely cool! Like the Liquify filter. It allows you to smoothly move the pixels in your image, almost without loss in quality. This can be particularly helpful if your lineart is shaky. Just correct if with liquify and no one will ever know!