I'm trying to implement a scene graph and in all the articles i've come across there is talk about object sorting. So you'd sort your objects by "material" for example. Now untill i sat down and started implementing it, i kind of took this for granted, because it made sense. But now i'm wondering what does sorting actually change?

In my engine, i have a manager for UBOs, i use those to store data that'll be shared between programs, at the moment that only involves time, camera and projection matrices and lights (i'm not worrying about managing which lights affect which objects ATM).

Now for each model i have to change the model to world matrix uniform, no sorting is going to change that. So is the jump from changing this matrix to also setting a material for each object that bad?

I vaguely remember reading somewhere that each time you change something in the pipeline, it has to get flushed and that can cause performance issues. But for each drawing call i'm setting up a model to world matrix anyway, so what sense does it make to ever be concerned about this?

BTW is there any information about whether changing a uniform and calling glBufferSubData is more (or less) expensive.