The Brussels-Capital Region contains nineteen communes. Each has a council, a mayor and an administration, and each is responsible for a slice of what a resident experiences as the city: local roads, some planning decisions, refuse in part, civil registry, local policing arrangements. On top of them sits a regional government with its own competences, and above that the federal state and the two main communities.
The case against is easy to state. A city of just over a million people, geographically small enough to cross by bicycle, has nineteen sets of overheads and nineteen approaches to the same problems. Cycle lanes stop at commune boundaries. Parking policy changes street by street. Coordinating a single traffic scheme means negotiating with a series of independent authorities that were elected separately and owe each other nothing.
The case for is rarely made as loudly but is not empty. The communes are the level at which most residents actually encounter administration, they are small enough to be responsive, and in a region with two language communities and a very high proportion of foreign residents, a dense layer of local government spreads representation more widely than a single city hall would. Abolishing them concentrates power in a regional executive that would then be further from the street.