Does having an agent increase the chances my material will be read by a publisher?

Beginning Writer's Answer Bookedited by Jane Friedman

In today’s market, almost 80 percent of books that major publishing houses buy are sold to them through agents. Agents are experts in the publishing industry. They have networking contacts with individual editors and know better than writers what editor would be most likely to buy their work. If an editor receives a manuscript or query from an agent, that book has a better chance of getting considered first—the editor knows the agent has already weeded it out of her slush pile and that the agent wouldn’t send something along that wasn’t at least the genre or type of book the editor was seeking.