Websites age. Software needs updating, certificates expire and security issues are found in code that was fine last year. Left alone, a site gradually becomes more fragile.
Regular care keeps small problems small. Backups mean a mistake can be undone in minutes, monitoring tells you about an outage before your customers do, and routine updates close the gaps that attackers look for.
Compared with the cost of an emergency repair, lost enquiries and a rushed rebuild, a modest monthly plan is usually the cheaper option, as well as the calmer one.