There are several reasons why enabling different groups to work on different pieces is important.
Modularization leads to specialization and the creation of disciplines (Specialists create knowledge within fields.) The danger of modularization is that the interfaces will decay. This is why Program and project management is important and valuable.
Additionally, good program management can “hack” interfaces that are not super clean.
It is not always obvious where the lines between groups should be. There are also politics around who works on what. Some things create more prestige or money if you work on them so groups will naturally try to take over some things and avoid working on other things. Defining clear timescales and scopes for collaborations is a tool to minimize politics.