Part of what makes Social Media so effective is that it's an interactive forum. To be successful you have to engage in the conversation. SocialOomph makes that easy because you can monitor all your Social Accounts on one screen.
Now here is where the pro account really shines. You can not only monitor your @replies but also your DM's (this is all Twitter speak by the way) and customize what you're looking at. On the free account you can only monitor @replies. Still pretty useful but not as comprehensive.