The simple answer is that it facilitates jumping to the point of interest. But what if entire book is point of interest? So, per me, this methodology gives vivid appearance of the content.
Index reflects school of thought of the author. Categorization gives vivid appearance of the concept as it highlights the thought process to get to the minor details of the concerned topic. You cannot systematize your knowledge of history unless you categorize as ancient, medieval and modern.
Long conversations (technical or philosophical) usually arouse boredom unless you converse categorically. What I mean is that before you begin your conversation which is intended to be long and intricate and you don't want audience to just give you their attention for formality, then it is best to first categorize your conversation and then give brief details of each category. Leave it up to audience to request elaboration on the particular category. This will invite audience to analyze categories and then get to the details of conversation avoiding boredom. I learnt this when my long technical discussion was greeted with confusion due to lack of categorization prior to conversation.
This art also helps in prioritizing phases and tasks in life. Every task in life should be categorized and dealt with expected priority. Prioritizing category is an outcome of consultation with wise personalities. Working in IT project, the categories of tasks could be project delivery, knowledge enhancement and employer compliance. Obviously project delivery should be highly prioritized.
Also Indian philosophy categorizes life as Brahmcharya, Grihasth, Vanprasth and Sanyas. Not looking at these categories only from orthodox point of view (philosophies of moving to forest during Vanprasth and so on) I think this is the systematic approach cited by erudite personalities which helps in getting vivid appearance of the phases of life. It helps in prioritizing the activities in each phase of life.
Categories of my blog para-wise are Introduction (Para 1, Para 2 and Para 3), Categorization in speech (Para 4), Categorization in life (Para 5 and Para 6).