Responsive web design allows a single website to alter its layout dynamically to suit the user's device being used to view that website. Large monitors (like a desktop) can display a website design with multiple columns of content while a small device (like a mobile phone) can have that same content presented in a single column with text and links that are appropriately sized to be read and used.
In the past, web developers built more than one site in order to accommodate different screen sizes. With the number of devices drastically increased we now develop a single website with a fluid and flexible layout which adjusts according to the screen size.
A large part of world's internet users are now using mobile devices. According to the 2018 Global Digital suite of reports (from We are social and Hootsuite), 72% of individuals from Canada and U.S.A. use mobile phones.
76% of Canada's population and 78% of U.S.A.'s population uses a smart phone! The above stats makes it clear that having a mobile responsive website is not optional anymore - it's a requirement!