Mobile-first design is a strategy for designing your product, focusing on the mobile experience and desktop/laptop designs follow. There is a solid ground for such a strategy since more and more users prefer to browse sites on their smartphones and even Apple watches allow this. So, you can’t ignore too many mobile users and adopt a mobile-first design philosophy when you take care of smartphone users first and then think about the needs of all other users when the mobile version is ready.
Of course, you can have a different viewpoint - the end-user should get an incredible user experience, no matter what device they use. It might be a good idea to research first what experiences are the most important. And in case you make sure that your target audience needs a mobile experience first - then the application of mobile-first design is justified.