Responsive by default, fast, user-friendly, modern lists with advanced filtering and content management capabilities, automation using flows, document cards, sites backed by groups, and more. You don't get any of this if you use classic SharePoint sites. If you want this and more you should use modern sites.
Classic SharePoint sites are slower, harder to use, more difficult to control, and more prone to security vulnerabilities. Modern sites don't have any of these issues. They're based on the latest innovation from Microsoft. There is no real reason for you not to use modern, especially since it doesn't cost you anything extra. To tell you the truth, it costs you even less.
Are you on modern yet? And if not, what's holding you back?