Social Media managers are important because they can handle issues anytime they arise, not just in the hour or two a day you might have allotted to handle your social media yourself. Customer questions? The social media manager can respond immediately. Want to send a tweet after that awards show, or during the final seconds of that game? The social media manager can make that happen! Social Media Managers can handle this because they aren't doing it in addition to a full work schedule, like most small business owners who try to do it on their own.
Social Media Managers are important because they know your target audience. A good social media manager will not only get you "followers" or "likes", they will get you the RIGHT ones. They will engage with the target audience of your business, not just anyone and everyone. They are able to figure out what really speaks to your consumer, and play on that by engaging them with that sort of content. A smart social media manager knows not to over-saturate with the same types of posts, and not to annoy consumers to the point that they are "unfriended!"
Social Media Managers are important because they are, by all means, the voice and face of your company. If a customer has a bad experience and wants to let the world know, they are going to blast it all over Twitter and Facebook. A good social media manager can diffuse these situations and get them under control. A good social media manager can provide customer service (in a private setting such as email), and take the problem out of the mainstream. The social media knows exactly what people are hearing, saying, and thinking about your business, whether it is good or bad. That is important info for you to know!