Often content plan and content strategy are seen as synonymous. However, that isn’t correct. A content plan has a content strategy.
A strategy is the how, when, and why of the content plan; the tone of voice, how much content for products versus services, is location important etc?
Which service is the most profitable, what is the company best at etc? How does our new content serve the aims of the business?
One prominent business objective might be to gain more customers from Google via the website.
So part of the overall content strategy could include monitoring and organising content so that it correlates with the difficulty and attractiveness (to the business) of a #1 spot for a keyword in Google. Without a strategy, the content can just become targetless and hence ineffective.