Today, the web is not only limited to desktop. According to Statista, there won't be less than 2.9 billion smartphone users by the end of 2019. With the "ever increasing" number of smartphone users daily, more and more people are browsing internet solely via smartphones. Yes, more than 60% of searches online now come from a mobile device.
Can you remember the last time you visited any website on your smartphone and shut that website simply because it was not looking good on the screen of your smartphone? It was likely not a pleasant experience. What if that was your website? You would be feeling nauseous about it.