Like basically everything in film, it's to do with expectations and prior experience. It's the reason high-frame-rate stuff looks weird or video-gamey or whatever, because our entire cinematic experiences have been at 24FPS not 48FPS. We have certain expectations of what happens when there's an explosion near a camera (it flares out, the footage is over-exposed and the camera shakes). If these expectations aren't met, you start to think there must be a reason.
But these expectations can be used for thematic purposes. To take the above example, if you have a group of WW2 soldiers next to a burnt out car and a tank shell explodes near them, the camera might go mad, then a grenade goes off on the other side of the car and the camera goes mad again. Swap round to the perspective of the tank and watch it fire another shell. The body of the tank recoils and the camera shakes again, demonstrating the power. Then a grenade goes off next to the tank, but the camera stays static. They're reinforcing the idea that a) the tank is reinforced, powerful, well defended, immune to small arms fire and even grenades but b) the tank gun itself is incredibly powerful - all through the deliberate choice to use or not use camera shake. And they do it all without you "thinking" about it. You don't need to go through that analytical process - your mind just absorbs this information, like character theme tunes or associated colours.
Lens flares can be used similarly - they can be deployed in various situations to present thematic ideas to the audience. The camera's lens can represent the character's eyes and be blinded by a search light hitting them, or they can be the mis-en-scene representation of a glimmer of hope in a perilous situation. Chromatic abberation is actually rarely used intentionally and, for certain lenses (And therefore cinematographic choices) it's actually inescapable even with the best lenses, but it too can be deployed intentionally to create certain moods - whilst you might want the invading army to appear disciplined, professional and organised (no CA), you may wish to give an emphasis to the fact that the rag-tag group of resistance militamen are using home made weapons and supplies (in the form of giving the shot a slightly home-video-camera look).
And finally, these things can all help blend CG in really well. It's often said that created good VFX isn't about making a render look photorealitic, it's about making it look like it was captured by a camera lens. This means matching to what's on the backplate, which often means that this stuff needs to be removed, only so it can be added back in afterwards. As the camera stares directly up at the desert sky with our heroine lying on her back, exhausted having found no water, the bright, hot sun (emphasised by a lens flare, natch) in a deep blue, cloudless sky is all the camera sees, the shot lingering to demonstrate the expansion of time in his hostile environment without water. Then, lo, the robot that we thought had died at the start of Act 1's head moves into camera shot, staring down at his creator with worry on his expression. As he does so, he blocks the sun (demonstrating immediately the idea that he's here to protect her, by standing in the way of the sun and her pathetic human body) and over the few frames it takes for his head to move over the sun, the flare shrinks correspondingly, simultaneously reinforcing the theme of rescue and helping to cement the CG robot's existence in the physical world.
