There may be multiple answers to this question and that is perfectly normal. Spirit Keepers do so for companionship, to learn about different worlds & periods of time, to strengthen natural abilities, to develop extra abilities, to learn about spell casting, the paranormal & magic and a host of other reasons... the primary of which is just naturally being drawn to them.
Some work with spirits to enhance gifts such as psychic power, clairsentience, and other forms of mancy. Others work with spirits to learn about different periods of time and other realms in their pursuit of discovery. Some work with spirits in complement to their spell casting & magic because spirits can offer insight, assistance & support through energy & power. And most do so to share life & company.
The practice of this branch of the paranormal is based strongly on Spiritualism and being spiritual should be not confused with being religious. Those who practice Spiritualism come from many & varied backgrounds of religious subscription, or lack thereof.