Website maintenance also often includes the necessity of repairing sections of your website in the event that they no longer function properly based upon ever changing technologies, updated and newer web browsers, mobile web applications and more.
Website links and graphic images need to be checked on a regular basis as these may need correction over time and as well the basic HTML pages may also need recoded as web browsers continue to evolve and what looked great in a particular browser yesterday might not look so great today.
The worst thing possible with a website is to set it and forget it unless you have no intention of acquiring top rankings in major search engines or continuously increasing your site traffic and site conversions or remaining topical to repeat visitors.
And more recently, major search engines such as Google are becoming very particular about web page load times, on page code and other factors- all which can effect your websites ability to rank at the top of the search engine.