ofFbo and glReadPixels: why is glPixelStorei necessary?

advanced
#1

This one is for the OpenGL savvies.

I’m trying to read pixels from an FBO and I want to understand why this does not work:

ofPixels pix;
// allocate pix to match fbo

fbo.bind();
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0); // <- This is because I'm working with PBOs, probably irrelevant
glReadPixels(0, 0, width, height, format, GL_UNSIGNED_BYTE, pix.getPixels());
fbo.unbind();

While this does:

fbo.readToPixels(pix);

The first option gives me an access violation

[…]

Actually, this makes it work:

glPixelStorei(GL_PACK_ALIGNMENT, 4); // <--- Needed, why?
fbo.bind();
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
glReadPixels(0, 0, width, height, format, GL_UNSIGNED_BYTE, pix.getPixels());
fbo.unbind();

The glPixelStorei call is made within ofFbo::readToPixels, why is this call necessary?

Also, why is glGetTexImage preferred to glReadPixels when TARGET_OPENGLES is undefined?