Before starting any website project it is important to understand your users, to gauge which devices they are primarily using? If you are selling clothes on an e-commerce website it is more than likely your users will be scrolling on their mobile, therefore beginning with mobile makes more sense. If however, you are offering B2B services, it is more than likely that your target user is going to be viewing your website during their work hours on their office computer and would make more sense to build your website from a desktop base point. 

Starting a project with desktop or mobile in mind, ensures your new website will be optimised for use, but can still adapt to deliver a seamless experience across any and all devices.