Yes and no.

Much of this is legacy from the old fixed pipeline (glEnable/DisableVertexAttribArray were modelled on glEnable/DisableClientState and first appeared in GL_ARB_vertex_program roundabout OpenGL 1.5: the older calls go back to 1.1) and dates to a time when you might be mixing vertex array drawing with immediate mode drawing. So you'd use vertex arrays for your complex meshes and maybe glBegin/glEnd for your 2D GUI.

VAOs didn't exist then so you'd need to be constantly enabling/disabling arrays, otherwise your current draw calls might pull in unwanted state from your previous draw calls. Obviously you wouldn't be using the same vertex format for everything (a mesh might have had position, normal, colour and 2 sets of texcoords; 2D GUI might just have position and a single set of texcoords) so a subsequent draw with a simpler format but more vertices might overrun the pointers used for a previous draw with a complex format but fewer vertices. In software T&L days that was death: your program would crash (if you were lucky: worst case is that it would continue running for a while and crash later on in some completely unrelated part of the code). Hardware T&L is more robust but back then you couldn't rely on everyone having it.

With VAOs - assuming you use them the way they're intended to be used - there's still a need for Enable but it could be argued that Disable is redundant. An attrib array must have a default state and that default state is disabled. If you try to draw from an enabled array but with no pointer set you'll crash too, so disabled is safer. glDisableVertexAttribArray is still handy to have around in case you ever want to modify a VAO at some time. And of course Enable must then exist if you ever want to go back again and modify it again. (None of that would exist if VAOs had been specified as immutable, but the ARB don't seem to have liked specifying immutable object types until quite recently. So in order for VAOs to be mutable both must exist.)

Unfortunately GL does carry a lot of legacy baggage like this around. Things that sometimes don't seem to make sense today can turn out to have a perfectly reasonable explanation when you wind the GL_VERSION back a few notches.

All of this goes away with GL_ARB_vertex_attrib_binding of course, which you should be using if possible.

Hi mhagain, thanks for the answer!

Right, so you're saying it's useful if you are re-using a VBO from the same shader?

Unfortunately, I'm using OpenGL ES so I can't use GL_ARB_vertex_attrib_binding.

So say you're drawing a mesh and it's shadow in two passes. You don't have VAOs.

The vertex format for the mesh is position, normal, texcoord.

The vertex format for the shadow is position only.

To save on data overhead in the second pass you have the mesh organized into 3 separate buffers: one for each attribute in the standard pass.

You set up to draw the mesh:

glBindBuffer (GL_ARRAY_BUFFER, vb_positions);

glEnableVertexAttribArray (0);

glVertexAttribPointer (0, 3, GL_FLOAT, GL_FALSE, 0, 0);

glBindBuffer (GL_ARRAY_BUFFER, vb_normals);

glEnableVertexAttribArray (1);

glVertexAttribPointer (1, 3, GL_FLOAT, GL_FALSE, 0, 0);

glBindBuffer (GL_ARRAY_BUFFER, vb_texcoords);

glEnableVertexAttribArray (2);

glVertexAttribPointer (2, 2, GL_FLOAT, GL_FALSE, 0, 0);

Then you set up the rest of your state and issue a draw call.

Now you come to draw the shadows. Because you've already got the buffer and array set up for positions, all you need to do is:

glDisableVertexAttribArray (2);

glDisableVertexAttribArray (1);

Then you set up the rest of your state and issue a draw call.

Now, think about what you need to do when drawing the next mesh?

but if you are drawing a shadow you are using a different shader. I thought once you switched shaders everything is reset.

Nope, you can switch shaders and most states will stay the same.

Nope, you can switch shaders and most states will stay the same.

Right, I didn't know that. I presume switching the frame buffer doesn't effect it either?

States pretty much live in isolation unless it's documented otherwise.