Most software developers can bash out code that is "good enough". Using the techniques on this website, you can develop software that goes well beyond that, including web applications that look and perform the same across the most-used web browsers.
Software changes over time as your business grows or changes. You need your software to be scaleable and maintainable. Will your software cope when your business expands and you suddenly need to deal with millions of records in your database? How easy is it to add new features or modify existing ones?
Making your site accessible to people with disabilities is required by law in some countries, but anyway, why would you exclude potential customers from using your website?
Finally and these days most importantly, security: The bad guys are out there. If they can exploit weaknesses in your applications, your data is at risk... and also your reputation. Your customers rely on you to protect their data; letting them down is not an option.