Obviously, with this method you need to write query letters, submit manuscripts, get an agent, and it make take up to two years before you see the book finally published, if it is accepted.
You also lose a great deal of control over the editorial process, book cover design, pricing, and possibly even the title. A major publisher may also want a detailed marketing plan as well as someone who is willing to be the "front guy" to do book signings, appearances, etc.
A major book publisher will also attempt to take all rights associated with the book within the body of their contract.
Book advances are rare and your royalty will probably be no higher than 10% of which your agent will get 15 - 20 %.
Your royalty is based on what is left over after the book is sold wholesale to book stores (40-50% off), distributor costs (15-20%), and again your agent's cut (15-20%). If the book, for sake of argument sells retail for $10.00, your royalty after everybody else gets their share will likely be 10% of $2.00 or twenty cents per book.
You will need to sell a significant number of books to make this a financially viable choice in this book model.
Some of the advantages of traditional publishing are the big guys have strong distribution networks and the cachet of being able to say you have been published traditionally still has a strong appeal.
E-mail me if you have additional concerns about traditional publishing.