Engagement rates are used to determine the success or failure of content on websites and pages that are written to gain customer base. When the goal is to get more customers from uploading relevant content, sites will review the level of reads and the amount of people who clicked on the article who went on to order anything.
If there is a low engagement rate on a piece of content it would be changed. The goal is to gain the highest engagement rate possible, 100%, although this is not possible in practice. Companies are able to compare their success against their competitors on social media sites through assessing the amount of times the information was shared or the amount of commentary that was attached to any certain piece.
The engagement rate is calculated relative to the number of users on any site, so a small site and a large site can have comparatively similar rates of engagement even though one site has exponentially more reads.