To my understanding the purpose of a Topic more so today is three purposes especially regarding a search by a user: (1) for stepped Topic Searches at HP (2) for the URL displayed at the Google Snippet (Title, URL & Summary), and also (3) a guide for the search process itself. That occurs with a user at Google and too at HP internal and their Search Engines.
Some TMI follows though may be of interest . . .
It is easy for some and others challenging as a task. There are 22 Main Topics. Those alone are a topic with there own hubs not further classified. I downloaded once in August out of curiosity questioning topics and Hub counts at HP. A very laborious process I did to kill time once. Total topics including those Main Topics are 7,157 with then 804,409 Hubs each with a Title.
Then there are subtopics. One may deduce the Main Topic is a subtopic itself when not further classified. There are a total of six levels with the Main as first (Level A). As Level A there are a total Hubs of 289,331. It is followed by:
Level B - 364 Topics at 266,296
Level C - 4,828 Topics at 195,598
Level D - 1,532 at 42,284
Level E - 385 at 9,157
Level F - 26 at 746
Each Main topic has its own subtopic structure for Hubs. If there were no subtopics then all Hub Titles would fall in the Main Topic making discovering a Hub near impossible. Why?
The total Hub count for the first Main Topic - Arts and Crafts, as a whole has 30,361 Hubs. That is a lot of titles to walk through. So, offered is the subtopics to further delineate a Hub from others by Topic. Even though the Main Topic may be the most appropriate, there is opportunity to narrow competition between Hubs for users searching. Skimming through the spreadsheet created as a generality specific to the first 3 levels the Hub count is near even.
There is opportunity to explore classifing a Hub at three locations - At the Hub Tool, at the Topic Tree found here http://hubpages.com/topics/tree and is also found at the Answer Section Tool. Personally I use the Answer Section as a tool for my Hubs before creating a Hub in its tool.
I use the Question queue to type in Keywords seeing the suggested topics & subtopics. Next, I formulate a Title (Recommended character count 65-70) that gets me closest to the Main Topic/subtopics I desire with suggestions. Then I use the Answer window for add'l info for a question to create a Summary using its character count - 140 to 200. Finally, I copy/paste to my Word document where my Hub is.