Following up on what is Mobile First, now lets see why this is such a big deal.
First lets examine the direct reasons
Mobile devices are outselling desktops
This happened in 2011 long before the earlier prediction of 2013. This means that you have a better chance of getting new users to your site from a mobile phone rather than a desktop.
International markets have more mobiles than PC’s
Globalization is on roadmap for all internet companies but here’s the fact – developing countries are more hooked to their mobiles than desktop. Therefore, your website better work great on mobile. Your traditional website will look ugly and will drive new users away.
Get rid of two code bases
Some companies have realized this mobile opportunity 3-4 years back and started created m.dot sites. These are mobile optimized/specialized sites that will only render on mobile phones. They will have lesser network footprint and somewhat optimized for gestures. But your code and teams become bi-furcated. The specialized mobile team will only know of the high priority feature much later, usually through an angry customer. Or the backend enhancements desktop is using can’t be used by mobile because their team is so small and can’t handle all the work. This approach is not scalable, mobile first gives the opportunity to merge these teams and codes.
Search engine optimization
If none of the other reason resonated with you, this should convince your current bottomline $$. Every web company wants to be found in search results and do everything in their power to optimize their pages for being found by google and bing. Guess what google started doing? They are highlighting mobile-optized sites in their search results. I am sure others will follow suite.
There are also significant indirect benefits by adopting mobile first strategy. These will come to your whole web ecosystem including desktop, tablet and mobile because of these redesigns.
For mobile devices network is precious and you have to carefully think about every KB of data that you are passing down to mobile browsers. This means your images gets trimmed down, font sizes reduced, high reliance on CDN’s. All the performance best practices that you couldn’t convince your team running on enterprise level ethernets will be extra visible on their phones. Once you start optimizing for mobile, desktop site will reap its benefits as well.
Clean and focussed UX
On mobile, screen real estate is premium. Granted phones are getting bigger but they will never have the size of desktop screens and they will always be users who like small phones. Which means there is not much space to put your divs/widgets. There is no spacing to be wasted. All the crap that has been shoved on to the main home page because of varying stakeholders inputs and legacy functionality, had to be rethink. Focus becomes the central aspect of UI. This is inevitably a win for the user and their experience.