You would essentially want to pin a post in WordPress to the top of your blog in the following cases:
1. Display Pillar Content on the Top of Your Blog
These are usually long, in-depth posts that support the rest of your content. Pillar posts tend to get the most views and shares, but their popularity may die down after a certain period. You also tend to link out to the other pieces of your content from such posts.
As you want to get the most views on such cornerstone content, you may want to display it on the top of your page. It would be a fantastic way to introduce new visitors to your content.
2. Show Announcements that Everyone Can View
Got a fantastic new affiliate deal you want your viewers to see?
You can make a post explaining the details of the deal and pin it to the top of your blog. And you can always choose to unstick it after a certain time period.
Sticky Posts are a fantastic way to enable more of your WordPress website visitors to view product updates and giveaways.