A mobile first approach doesn’t just concentrate on developing for mobile phones, it is also used to develop better usability of sites, develop better use of Web real estate and better reduce the amount unnecessary elements from pages.
With Mobile First, we are in a process of developing adaptive, future-friendly sites. This progressive enhancement helps in keeping the content and media featured on a site from being far too heavy for the lowest-tech phones to handle. By developing with the lowest common denominator, you can keep all your users happy and navigating freely through your site without the complications from those pages that are geared for more media heavy and larger tech-minded. Even though we start with the most simplistic core foundation of a site, to progressive add features to each level of higher tech, larger screen, etc. the process to do so is much easier.
Along with the technical positives, from a design perspective, a mobile first approach helps us develop for ease of usability. With, roughly, 80% of the screen size taken away when you start with mobile first design, you have to think about how to utilize your space in a much more conservative manner. This helps in keeping the core values you want to present to the users in the forefront without flooding with superfluous amounts of filler. Then, as you progressively enhance for larger and larger devices, screens, etc. it helps keep you focused on the core values of your site and what the user can accomplish when visiting.