You have a legal right to hold the copyright yourself. You could even produce a reasonable looking camera-ready manuscript, complete with graphics, especially with today's ubiquitous and reasonably sophisticated desktop publishing technology. You could even locate a local offset shop that could print your book and bind it. You could put them in your garage, slap labels on them and send them to all of your friends. Thus far in the authorial process, you haven't needed a publisher.
Most authors decide they want a publisher for a whole host of reasons.
Most authors need one to market and sell their book. Many authors readily acknowledge that they would rather relegate the marketing and selling of their book to others. However, to be successful in penetrating all potential markets and exploiting the true sales potential for a book requires a partnership between author and publisher. As an author, you know your book's content intimately. You are in an ideal position to articulate the features unique to your book and what makes them beneficial for readers and adopters.