Topic lists link to articles on a particular subject. There are two types of topic lists on Wikipedia:outlines, andalphabetical article indexes. Outlines differ fromcategories found in Wikipedia's category system in that categories are usually limited to one level each in a tree, while an outline can be an entire tree, with many branches and many levels. This gives the benefit of having all the topics on a given subject in one place - this allows you to scroll down and read the whole tree, and every topic in that tree's subject. Indexes are single-level topic lists, with topics ordered alphabetically.
Topic lists allow for centralized management of all the links on a subject, as opposed to the decentralized approach of the category system.