Nowadays almost everyone uses a mobile phone for a variety of reasons. People make purchases, order taxis, and food delivery using their smartphones. That is why businesses have to adapt and develop mobile versions of their websites. 

The essence of the mobile-first approach presupposes the development of the Mobile Version first, and on its base build a Desktop Version, and has its pros and cons. 

Please read the article: Mobile First. The Essence of Approach and Why It’s Gaining Popularity to learn the details considering this concept.