I've read through a few contracts in my time. Nowadays, whenever you submit a story or an article of some sort to a professional publication, you have to expect such an agreement to be drawn up. What's more, there are no exact standards to any relationship between a writer and a publisher -- so such considerations have to get drawn up on paper, leaving us to the mercy of lawyers and their endless fine-print clauses.
I'll point out that it's clear as to what the writer receives with regards to such agreements. The most obvious benefits to professional publication should be as follows:
Your work in print. While this is not automatically guaranteed (as any number of factors may prevent publication), you can be sure that any publishing company that acquires your work usually won't end up sitting on it.
Compensation. I've described the different kinds of payment you can get in return for a story back in a previous post; Suffice to say, however, that this will usually involve money, tangible objects, or future services (hopefully involving dinner).
Future work. Some publishers will agree to contract you for more writing jobs, whether this will involve journalistic articles or editing, or the acceptance of further pieces of literature. Even if a publisher doesn't specifically do so, however, the fact that they now have experience working with you might make them more favorable to your future work submissions.
Corporate backing. While this is not too obvious, selling the rights to your work to a publisher effectively gives them a stake in your affairs. This will be effective with regards to bringing your work to a broader audience, or securing further legal representation: It's much more difficult to get away with plagiarism if the work in question has a good-sized corporation with some very vested interests in it.
What I feel is not entirely clear, however, is exactly what the publisher usually wants -- and gets -- from writers. Different publishers will usually want different things depending on the length of time they've been in the industry, the type of publication they're running, and the nature of business according to their own opinions.
From my experience, publishers seem to want any or all of the following items:
Publishing rights. This is the most obvious thing that the publisher signs for, and usually why the contract exists in the first place. This allows the publisher to turn out copies of your work for sale in various places, and win a certain amount of profit for its own efforts. Publishing rights like these are usually exclusive (i.e. you can't publish the same work anywhere else), and are usually reserved for a fixed period of time. Generally, the greater the degree of exclusivity or the longer the length of time, the higher the compensation for the writer.
Ownership of your work. Publishers can also buy your work completely, in which case they'll own pretty much all the rights to it in exchange for what's hopefully a good amount of cash. I expect that this is usually the case when it comes to ghost-writing affairs, but I wouldn't discount the enthusiasm of a few publications.
Guarantees of originality and quality. This should go without saying, really -- a publisher will not secure the rights to anything that was copied or sourced from another person's work. Most publishers will, I think. Probably. Well, at least more than a few.
Exclusivity. Sometimes exclusive rights to a single story aren't enough. If you're a talented enough writer -- or if you've got a ravenous enough publisher -- a writing contract may require that you give up any future obligations to other writing venues. This isn't as bad as it sounds, of course: It does mean that you're being held in a certain regard. Some people will prefer writing in a more open environment, though.
Right of first choice. A few publishers will go as far as to demand the right of first submission, one that requires you to submit your works to them for consideration before offering them to other venues. This is a little better than the notion of exclusivity, in that you still retain the possibility of working for other people. Another positive aspect is that the publisher is almost certain to provide a home for your writing in this way. The catch, however, lies in the inconvenience: Some people won't be comfortable with the idea of waiting for a story to be rejected before being able to use it.
Your eternal soul. Just kidding.
Exactly whether or not a writer agrees with these requirements is a question of how much each person is willing to accept. The great debate probably lies in how far we're willing to be beholden to our publishers, and how much freedom we need in order to do our writing. Some people prefer a freelance status, while others look for a certain amount of governance to their actions. We'll even have to admit -- figuratively, that is -- that some people would even be perfectly willing to sign over the last item on that list above.
When in doubt, I suppose, we could just read the contract to begin with. Then there's always the question of negotiations to fall back on. The beautiful thing about contracts is that, during their formulation, they're not entirely written in stone quite yet -- it's pretty much up to us to be flexible enough to adjust them as we see fit.