A website is never really "finished"
There's a common belief that a website is a one-time project: build it, launch it, done. In reality a website is software, and like all software it needs ongoing care. The web changes constantly — browsers update, security threats evolve, the code your site relies on gets patched. A site frozen in time slowly drifts out of step with the world around it.
Security and maintenance is the discipline of keeping the site healthy over time: up to date, backed up, monitored, and protected. It's unglamorous work that you mostly notice only by its absence — when it's neglected, things break, and sometimes they break spectacularly.