Years ago the technologies to develop web projects were static enough so that web maintenance of these were infrequent. We would be talking about an investment every 2-3 years, or more according to project.
Nowadays it is different, since the current tools allow a continuous and changing evolution of new web applications, calling them (from the ICT sector) "live projects". That is why its maintenance is carried out during the same year.
These live projects are integrated with databases, CRMs, ERPs, frameworks, plugins and the like that are constantly evolving. For example, it may be the case of the introduction of a new parameter in CRM or ERP, a new calculation method that improves the web application, or a security update. It is important in these cases that the web project receives maintenance and obtain new value immediately.
The web maintenance team behind you will make a difference in the adaptability and evolution of your web application, either in specific aspects or in long-term developments.
In custom-made web projects, such as an educational platform, an extranet or a management tool, there will probably be small usability details or functionality that you want to add: insertion of a new search tool and filters, integration of data from External origin or simply change the distribution of the interface to increase usability. All these possible changes, both small and large, require a professional who will perform such web maintenance tasks.