Why you need to maintain your website on a consistent basis is to make sure it is secure from threats like spam and malware; to keep it updated so that it maintains all functionality to your current audience and new visitors, and to keep it current in SEO practices to attract and retain customers.
Most of us know that technology changes fast, and it changes rapidly in website design. These changes (updates) effect the basic functionality of your site. If a link is broken, or sections of your site loads slow, or not at all, you lose your visitor. Most often they will not return to see if you have fixed your site.
In 2019, your website’s security is the biggest reason to have your website up to date. Security ranges from spam, viruses and malware, domain registration privacy, content plagiarizing (which effects your search results), and the protection of data being passed between your website server and the user’s browser (SSL).