Publishing is a hard and very competitive industry and, upon numerous rejections from traditional commercial publishing houses, wannabe authors who do not feel that they have the time and business acumen to self-publish may turn to another outfit to help them release their books. But beware of vanity presses; they are predatory and take advantage of the vulnerability of writers who are desperate to see their words in print.
Remember traditional publishers produce books; they are not service providers. They take on any financial risk associated with publishing a work. Vanity publishers upend this model; they don’t care and are not selective about the quality of a manuscript or its possible appeal to the market. That’s because their income stream derives from selling writers the dream of publication.