Designing desktop-first and then squeezing it into mobile is backwards. Mobile-first forces clarity.
Constraints breed creativity
When you only have 375px to work with, every element must earn its place. Mobile-first designs are leaner, faster, and more focused.
Performance is a feature
Mobile users are often on slower connections. Building mobile-first naturally leads to performance optimisations that benefit all users.
Progressive enhancement
Start with the core experience on mobile, then add layers of richness for larger screens. Not the other way around.