Lists on Wikipedia have three main purposes:

Information:

Lists are a valuable presentation format, especially the structured list. Examples include lists organized chronologically, grouped by theme, glossarized, or annotated.

Navigation:

If a user is browsing without a specific research goal in mind, they would likely use the See also lists embedded in articles. If the user has some general idea of what they are looking for but does not know the specific terminology, the general topics lists (outlines and indices) would be most useful.

Development:

Outlines and indices give an indication of the state of the 'pedia, the articles that have been written, and the articles that have yet to be written (shown by red links).

Like categories, lists are great for keeping track of changes to subject areas, using the Related Changes feature. Unlike categories, lists are centralized, providing direct control over the contents. Lists also allow detection of deletion of pages from them (the list item will "red link"). Another advantage of a list is that changes can be tracked in the page's history, while changes to categories cannot.