Historically, most designers have started with the desktop side of any web project and worked their way down until they reach mobile. This practice is tried and true, and allows your clients to visualize their website in a large scale before imagining it in the palm of their hands.
While this may be the case, it's becoming more and more prevalent that people are ditching desktops and viewing everything on their smart phones. Designers are creating beautiful websites that are built to function properly on desktop, but they leave mobile as an after thought. This could derive traffic from your mobile site. Which brings me to my point, is mobile-first design the new process?
Let's revisit the importance of responsive design, shall we?
I can't count the number of times I've viewed a website on my iPhone while sitting on my couch versus sitting down at my desk, turning on my computer and then pulling up the website. That being said, it frustrates me when websites are NOT responsive and I have to zoom into the screen to see any of the navigation or content and by then I have a headache and I completely forgot why I visited this site and I'm moving on to the next one that might make my experience a little less, annoying.
Mobile and responsive isn't a niche or a trend, in fact, it's not even in the future - it's the present. There are over 1.2 billion users on the web worldwide, in the U.S., 25% of web users are mobile only (meaning they almost never touch a desktop or they don't have access to one). The craziest fact of all, is that there is a strong possibility that most people will only see the mobile view of your website.
If your website provides services such as E-Commerce, you will want your customers to find it easy and painless to visit your site and successfully complete a purchase. Especially if that user is mainly doing these purchases through their mobile device.
By utilizing mobile first web design, you are designing your layout with friendly UX in mind, which only helps when you move on to tablet and desktop. It can be difficult to design a website for desktop with certain functionalities that may not work properly when the website is scaled down for mobile. This also ensures a clean design throughout all platforms.