The first reason is, you want to see what is the most popular so you can write a similar blog post, or post an image within the same subject. This will keep people engaged with your site, pinning more images, and driving traffic back to your website!
Secondly, let's say you want to find out what people are pinning from your competitor's page. This will tell you what interests people in your target market have. You'll then be able to tailor your marketing messages to meet the demands of your market. WIN-WIN.
All you have to do is enter the following URL:
Mine would be: http://www.pinterest.com/source/foodwinesocial.com
This works for any website. Go try it out and let me know how it goes!