primitives are good as hidden collision objects, for example it would silly to make, load and hide a box object when you can just make one inside the code, so in the case of a door trigger you would just use a primitive box in front of the door (hidden) and then when the player hit it open the door, or use a hidden cylinder to detect if the player walked into an arena that then locks and releases a monster, etc, you can also use primitives to make up simple puzzle levels etc, or to act as placeholders inside WIP projects.
as for a matrix, they mAke simple ground objects and can be deformed in real time, larger ones have quite a high overhead in respect of the cpu time they consume, but smaller ones are ok, you can use a simple trick to make animated water with them as well.
we know where area 51 is, but where are areas 1 to 50?, and what do they do there?.
