Okay, first let's answer the question based on what subrights commonly mean in the publishing industry NOW.
Subrights are the permission to use the content from the primary license the publisher has purchased (almost always the book) in subsidiary forms. When a publisher buys the right to publish your book, that company usually secures all subsidiary rights in the deal. This allows him or her to exploit these secondary rights him or herself, or more commonly to sell these rights to others to create new products that disseminate the content and generate new revenue streams.
Common subrights are film and video rights, audio books, workbooks, gift books, e-books, translations, book club editions, international editions, commercial rights, gift products, and according to the contract language of many publishers, any medium that now exists or that will exist in the future in the universe. In other words, anything that can house your words.
The basic reasons that publishers secure subrights, the right to re-license what you've sold to them, are:
a. Most book publishers, not surprising, are very good at creating books, but also not surprising, not as good at creating other products that expand the reach of the content, like motivational coffee mugs or Lithuanian translations or motion pictures. But they do have staff or have contract workers who can find companies that do those things very well.
b. Since the publisher invests significant money into taking a book to market, with no guarantee that the book will be profitable, his or her default position is to reserve all opportunities available to earn a return on that investment.
So does the publisher get all the money? Not unless you signed a bad deal. The standard contract terms is for publisher and author to split the proceeds.
Why should I give the publisher all these rights? Don't if you don't have to. But unless your name is Stephen King or John Grisham, it's probably going to be a deal breaker for the publisher. And if you have no history of selling subsidiary rights, why hold onto them? If you have a compelling argument on why you can outperform the publisher - i.e. Ridley Scott has already bought an option on the screenplay adaptation of our work - or you know your publisher doesn't attend international events and has never sold a translation right - or your uncle owns a direct mail book club - then fight for them! If you think you can outperform the publisher, try to negotiate a time limit for the publisher to have exclusive right to sell subrights to your work - or counter his or her offer with terms that give you a bigger share of the subrights revenue if you generate the sale.
How important are subrights? For many publishers, their core business, creating books, is a break even proposition; profits come from subrights. For the most successful authors, creating a book opens up opportunities for many other ways to express their content, while making more money and promoting sales of the original license, the book.
Can subrights hurt? Sure. If you've written a motivational classic and the publisher sells quotes to an employee award company that makes really ugly plaques with your name on every single one of them, then yes, it can hurt you. If the sell of subrights doesn't generate new business but only replaces what the publisher would have sold anyway (cannibalization), then there's really no benefit.
With the proliferation of e-books in particular, the reality is that subrights might soon be the only thing you, an author, sells. In other words, the primary product will be the content and any expression of it will be the sublicense, including the veritable paper and ink book, which may or may not be necessary to distribute the content.
That's undoubtedly a long ways off. Or is it?
Oprah Winfrey just reported that the Amazon Kindle is now her favorite "gadget". That means the future might be closer than you think!