Simply put a website designed for desktop or notebook doesnt convert well to your smartphone. And becuase more and more users are browsing the web with their mobile devices a hard to read slow loading tough to navigate mobile experience is going to send them off to shop elsewhere.
There are a number of techniques that can used to optimize a website for mobile.
As wikipedia puts it:
” Responsive web design (RWD) is an approach to web design aimed at crafting sites to provide an optimal viewing experience—easy reading and navigation with a minimum of resizing, panning, and scrolling—across a wide range of devices (from desktop computer monitors to mobile phones).”
Make sure your website plays nice with mobile,