It can be useful, when you want to release your minigame for games webs,
or you do beta version of new version for your minigame and you upload it on another page, you want upload only your minigame, not all game (it is unnecessary, beacuse players will be confused, when they see full game insteal only one part + it will use more disk space (when they download it…).
Maybe when you also want have it sorted, beacuse when you have all in one project, you have there too many maps and gamemodes, when you have more projects and hook it to one and release ex. 2 in one, it will be good solution.
If we can´t release only specific part of the project, or if we can´t hook one project to another project and in another project package will be another project + one project, can UE 4 developers reply me, if they can add that in next version?