A good publisher will help you market your game.
Marketing is all about getting people to know about your game. Your game may be up on facebook, but how will anyone know about it?
Marketing may be a combination of press releases, paid advertising, the setting up of interviews with trade press or blogs. They may post to their twitter/facebook feeds which have tens of thousands of followers.
A publisher can also bring connections with the trade press, a big social media following and generally a bigger megaphone than you as an individual have. Not to mention specific skills and experience in getting word about new games out to the public..
Is it worth it? That depends on what real value the publisher brings to your game-- can they get you an interview with gamespot? Will they prominently feature your game on their site? Will the issue a press release about your company and/or your game? Those are the sorts of things you'd want a specific list of when making an agreement with a publisher for a self-funded game.
Marketing is hard work; sadly, with few exceptions "If I build a great game, people will just hear about it" isn't how it works.
I wholeheartedly agree with Tom-- try self publshing/self-marketing. At the very least, you will learn a ton about the whole process
Executive Director, GameSoundCon:
GameSoundCon 2016:September 27-28, Los Angeles, CA
Founder, Brian Schmidt Studios, LLC
Music Composition & Sound Design
Audio Technology Consultant