Teachers require it and students hate it. Besides, when will I ever use that in the real world anyway?…
When starting any venture, whether you want to launch a business or write a 400 page book on the migration habits of geese, doing your homework on the subject can give you the valuable information you need to make the best decisions possible.
In terms of writing a book, you need to research the demographic of your potential readers; those who will be the most interested in your topic. That will help you determine where and how to market and promote your work. Incorporating the “lingo” or terminology into your book that is used by your target audience will make your work relatable, interesting and worth reading.
If you are serious about self-publishing, do your homework on the companies whose services are designed to help you do that. Ask plenty of questions about copyright ownership, who is responsible for what, how much control you (the author) will maintain, how much it will cost to publish your book, and so on.
Other avenues that are often overlooked are industry related associations, retail establishments, social networks and the web. These too are great resources to help you do your homework. (And you thought “homework” had no purpose!)