Your customers are holding a phone
For most local businesses, the majority of website visitors arrive on a phone. Someone searches "plumber near me" at the kitchen sink, taps a result, and decides in seconds whether to call. If that phone experience is cramped, slow, or requires pinching and zooming, they're gone before they read a word.
Responsive design exists for exactly this reality. It's the practice of building a site that automatically reshapes itself to fit whatever screen it lands on — a phone, a tablet, a laptop, a giant monitor. One site, many shapes, always readable.