A vanity page is a page that a person posts for promotion of them-self, their company, their ideas, etc. Essentially they use the wiki as a free vanity press. A page whose content is off-topic is particularly susceptible to being viewed as a vanity page. Main space content is also often more closely regulated than user space.
Such pages are sometimes deleted as spam, but norms differ from one wiki site to another. Wikipedia clearly states that it is not a free web host, and that vanity content will therefore be deleted. For one thing, vanity pages are deemed to manifest a conflict of interest. Other sites are willing to be more 'Facebooky', and to welcome vanity pages as potentially useful user-generated content, so long as there is an actual human being who has taken enough interest in that particular wiki to post his content, rather than a bot advertising generic erectile dysfunction remedies. The idea may be that the user has an incentive to cross-promote the wiki as a way of drawing further attention to his content.