A responsive WordPress theme will automatically adjust itself to the user's screen size. This means that your WordPress site will look equally good on mobile phones, tablets, and desktop devices.
Around 55% of global internet traffic is on mobile devices. That means about half of your users will be accessing your website using a mobile device.
If your website doesn't fit their screen or looks ugly on their device, then they will immediately leave. They may never find your website again.
This is why you must always use a responsive WordPress theme for all your websites.
Responsive themes improve the user experience and also have a significant SEO (search engine optimization) advantage. Search engines like Google rank mobile-friendly websites higher in search results, which brings more traffic to your site.