Multiple outlets publication
Publishers have to eat too, and if you can take the same thing you published with them and publish it again with someone else, who will buy it from them?  Their argument is that they invest in the publication process and need to get a return for it.  I have some skepticism about this argument (these days, universities could pretty easily and cheaply operate open-access online publications, etc. etc.), and am personally a much bigger fan of open access, but that's the standard argument.

N.b. there's also a (smallish) benefit for authors in having a central place to go for things like reprint requests.

Also, many publishers these days are ok with a certain amount of republication.  For example, most copyright transfer agreements I've seen for journal articles explicitly allow the author to republish in a larger book, e.g., a collected papers book or a follow-up.