Time - While it's not tough to manage your own social media accounts for your own business, it is time consuming. Wouldn't you rather dedicate your time to working for new and existing clients? In this sense it's worth it to pay someone to manage a couple of your accounts.
Reputation - Now this also goes back to having time. When you hire a social media manager, those managers have the time to respond and also manage your reputation should problems arise with an unhappy client. It's their job to continuously check your accounts and messages and respond how you would like them to when customers have complaints, complements or questions.
Reach - With constant monitoring and updating, your brands' reach should grow consistently leading to greater marketing opportunities and sales. Your social media manager will work to increase your followers and fans in a targeted manner. This will create a lot of exposure for your brand and company.
Competition - There is a very good chance that your competitors already have a social media management firm updating and tracking at least two accounts for them(Facebook and Twitter). It's in your best interest at this point to at least consider doing the same, especially if you're business focuses on genuinely solving a problem that people have or if you provide a service.
...and last but not least:
Community - Social media's main focus is connecting people, building a strong community around your brand will ensure that in the future your company grows and stays relevant. Word of mouth is still the strongest form of marketing and social media gets people talking even if it's through their fingers on a keyboard.