Technology has empowered people to accomplish many things. Bands can put their own CDs out, people can create their own videos and upload them to the web, and authors can write books and self-publish them. The power is in the people's hands - who needs publishers anymore?
Well, we all do. Publishers have a lot of connections, resources, and abilities that neither our internal staff, friends, or the internet can provide. Ability is one thing, human effort and relationships is quite another. But before I go any further, this Scalzi post explains things quite sensibly and thoroughly.
There are certain benefits to self-publishing, but again, it depends on your purpose for writing and what you want the book to really accomplish.