Just like anything you do in the world of business or marketing, creating a plan for how you are going to do something is helpful. It offers a clear vision of your strategy, goals and expectations. Often, a multi-faceted plan is called a campaign, but just about any organized social media or marketing plan can be called a campaign.
Always ask yourself what you expect form a campaign?
Are you looking for more traffic? A larger community? More sign-ups? How about the most important outcome: a financial conversion? It is important to develop a funnel (marketing path) as part of your plan.
Social media is certainly an area of marketing where having a plan will keep you focused in a chaotic space. Time, money and effort can easily be wasted in social media.
Here is a good post on four important points to consider before launching a campaign.