Direct and indirect light are substantially different in the real world. The difference is in the colors it contains and the intensity. When light bounces off something, some of the light is absorbed and does not bounce off leaving the rest of the light to bounce. It can lose intensity, or luminance, across all colors - making it darker,
No. That's like sitting on top of a stack of boxes, following the forces your weight causes and concluding the 'direct' force you apply to the top box is somehow different than the 'indirect' force on the bottom box. This makes no sense - thinking like this you would never be able to develop a working rigid body simulator.
With graphics we get close to a similar situation. Our box of tricks becomes empty, "Reflections don't need to be correct - they just should look good" does not impress anymore, and hardware is powerful enough to do better.
By drawing a difference between direct and indirect light you get stuck in something that works now but may prevent you from thinking forward.
Probably, like most graphics devs you got used to this seperation so much that you forgot (or are not willing anymore) to put this in question.
Light is just photons, no matter where it comes from, what it have gone through or by what it has been bent.
And no matter if it has been emitted, reflected or refracted - because in practice it's always a combination of all 3 of them.
But there's a big difference between direct sunlight on an object and direct sunlight that has bounced off a red wall and is then bouncing off the object.
Notice that the mirrorball model explains all this wihout the need to seperate. It also explains all the math, which is surprisingly simple. (Simpler at least than reading a PBR book):
The photograph of the mirrorball shows a small bright spot - the sun. It's far away, so just a small area but still very bright (distance falloff)
If the sun moves out of horizon, on the backside of the normal plane, its area on the photo decreases smoothly to zero allthough distance is constant (cosine falloff)
The red wall has a large area on our photo, but its intensity is much smaller because it also 'sees' the bright sun only as a small spot and it absorbs any light that isn't red (simple wall BRDF).
Again - no matter if a photo pixel shows sun or wall, we sum up all pixel colors and take the average to get the correct amount of incomming light.