Some OpenGL applications may need to render the same object in multiple locations in a single scene. OpenGL lets you do this two ways:
1) Use ?absolute coordinates". Maintain multiple copies of each object, each with its own unique set of vertices. You don't need to change the ModelView matrix to render the object at the desired location.
2) Use ?relative coordinates". Keep only one copy of the object, and render it multiple times by pushing the ModelView matrix stack, setting the desired transform, sending the geometry, and popping the stack. Repeat these steps for each object.
In general, frequent changes to state, such as to the ModelView matrix, can negatively impact your application?s performance. OpenGL can process your geometry faster if you don't wrap each individual primitive in a lot of changes to the ModelView matrix.
However, sometimes you need to weigh this against the memory savings of replicating geometry. Let's say you define a doorknob with high approximation, such as 200 or 300 triangles, and you're modeling a house with 50 doors in it, all of which have the same doorknob. It's probably preferable to use a single doorknob display list, with multiple unique transform matrices, rather than use absolute coordinates with 10-15K triangles in memory.
As with many computing issues, it's a trade-off between processing time and memory that you'll need to make on a case-by-case basis.