1. There is only one URL.
Originally the desktop version of a site and the mobile version had two different URLs. With responsive design, the same URL is used and the display re-proportions itself based on the screen size (desktop, mobile, tablet). This provides a consistent experience for your user.
2. It requires less time to maintain.
Since you don't have multiple pages for the same content, your site doesn't require as much upkeep. You have more time to focus on other aspects of your business.
3. The site loads faster.
There is only one URL as mentioned earlier, so users are not redirected. Because there is no redirection, the website loads quicker. A faster site will lead to happier users and more conversions.
Optimizing your website using responsive design reduces the load time for your site, decreases the amount of time you must spend maintaining your site, and better meets customers' needs. Google responds well to responsive design since it doesn't have to crawl multiple sites. Using responsive design is likely to increase your ranking on Google as well as increase your revenue from the rise in conversions.