Responsive design ensures that your website is displayed correctly regardless of the medium used: computer, tablet, smartphone and regardless of the size and resolution of the screen.
For example, without responsive design, the site appears on three columns. By viewing it on a tablet or smartphone, the result may be less attractive to the user: some items could be hidden, others distorted, the size of the characters inadequate, etc. The impact is all the more important as the display can be done in portrait or landscape mode. On a smartphone, for example, a large image can totally destroy the layout. Finally, if the site is heavy, it will take a long time to download on a smartphone.
The responsive design concept overcomes these disadvantages. The site's design is responsive and allows the site to adapt to the browser window scale used by the user, regardless of the resolution and size of the screen. The dimensions of the site are calculated in percentages (relative values) and not in fixed values, so that all the elements contained on the page can be adapted. In other words, this means that, regardless of the medium used, all the elements of the site will display in the same proportions and thus facilitate reading and navigation.
The installation of a fluid grid generally allows sites to adapt to both portrait and landscape mode, but to expand capacity beyond the office, further optimization is required. Media (images or videos) are also scaled.
The usefulness of having a responsive site?
Today, one in two Internet users connects to the internet from their smartphone and tablet sales explode. Google Analytics can tell you the proportion of visitors who come to your site with a mobile device. You will see the urgency of transforming your site into a responsive web design site.
In addition, Google is the search engine we all use. They have made a tool available to test its website. There is no concrete evidence that Google penalizes or not a site that is not mobile responsive. On the other hand, Google advises to apply a mobile strategy in order to offer a pleasant mobile experience. The link to Google's tool is at the bottom of the article.
Google Mobile Test: https://search.google.com/test/mobile-friendly