REST Assured is powerful set of APIs to automate Restful APIs/Restful Webservices (just like Selenium WebDriver for Web Application). Almost all web applications use Rest APIs to communicate with back-end and it has gained a lot of popularity in the modern day application architecture.
It is very important for QA/Automation engineers to understand how to automate Rest API tests. Automating Rest APIs tests is very crucial for projects because Rest API tests are quick to run and more stable unlike web automation.