Responsive design makes emails render well on a variety of device screen sizes and email clients (Outlook, Gmail, Yahoo!…). Today, a large majority of emails are opened on a mobile device. If you don’t have responsive designs, you’ll be providing a poor user experience to those “mobile openers”, and they won’t be clicking on your email campaigns.

Choose an Email Service Provider (ESP) that offers a responsive email builder to automatically design marketing emails (as well as transactional emails) that display perfectly on the screens and email clients of all your subscribers. You can also take advantage of a tool like Inbox Preview to make sure your email templates look good everywhere. This will help you dramatically improve your click rate.
With a responsive email builder, you won’t have to worry about things like media queries or CSS. The email builder will take care of these things for you. However, there are other elements that might interfere with the end result, which you should keep in mind:
Result
Responsive designs get a 24% increase in mobile clicks vs non-responsive designs. This is why using a responsive email builder will help you increase your emails performance.