We update website core system, themes, plugins and relevant code on a monthly basis and immediate need (as security reasons) to ensure complete site health.
The website is the presenter of the business. When it's not professionally maintained, it is seen and it reflects badly on the capabilities of the business. But this is not the reason why websites are maintained. The main reason for website maintenance is of course security.
A well-maintained site works better, looks better and of course produces more results but above all does not require the preoccupation of company owners and busy staff.