Development is the work of designing and building the site: structure, design, content implementation, functionality and launch. It has a defined start and end.
Management is what happens afterwards: content updates, technical maintenance, monitoring, fixes and improvements. It is continuous and it is where most websites quietly fail, because nobody was assigned to it.
The common pattern is a business that invests well in a build, then leaves the site untouched for two years. Contact details drift out of date, plugins age, performance slips, and the site slowly stops representing the business.
When commissioning a project, decide who owns management before launch — an internal person, your developer under a support arrangement, or a dedicated management service. The decision matters more than the technology.