Websites are like babies: they require care and bringing up. A successful website is recognisable by the number of visitors that become clients. To achieve this, it needs constant improvement and reinventing, both in terms of content and functionalities.
Technically, regular back-ups and security checks to prevent any breaches are needed. Traffic needs to be monitored and analysed. The CMS, if one was implemented, needs to be constantly upgraded to benefit from improvements and security updates. Alternatively, security gaps need to be monitored and solved.
Maintenance never ends.