Similarly to how your website needs frequent updates for security and to keep it in the best shape possible, your website's content needs a plan for it's upkeep.
If you're thinking this means planning on what you need to write for your company, you're right, but only partially so.
You see, content strategy is not only what you write, but when you write, who does the writing, and the strategy around keeping everyone up to speed on all things content. So your employees never have to wonder what content is doing what.
And, finally, the all-important measuring of your content.