A thesis can be a gold mine to someone who is delving into a new area of research, a subject that has not appeared in published form recently or at all, or to someone who is just having trouble locating information.
Not only do theses contain the text of someone's Master or PhD level dissertation, they also include extensive bibliographies and will often list relevent databases and journals for the subject.
If you're having trouble finding information on your topic, try searching in theses.