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Yes, as your question for 700 posts to write descriptions for all those I would say yes. For your website overall ranking you must write the description for all page uniquely.

I think Yext is really worth the cost and has the most important local directories covered. One main advantage of services like Yext or Moz Local is the ease of making updates to the listings. With manual citations, if you need to make a small change to the hours or description of the business, you need to repeat the task for all the sites one by one. But with Yext or Moz Local, it can happen quickly by just updating the business under your Yext or Moz Local account while automation will take care of updating the rest of the listings in the background for you. There is also this business listing scan tool out there that I thought might be relevant to this question, it covers the most sites and also tells you what's your completeness score and your accuracy score with a short link that you can use to share the web-based report with others. I hope you find my answer useful.