Lens flares boil down to being an optical mistake in cameras. When photons travel through a lens, depending on the lens itself, you might get extra light from an unexpected angle. Photons inside the lens itself could also bounce around in unexpected ways because we, as humans, cannot yet manufacture optically perfect physical lenses. Another source of problems is if multiple lenses are in any way misaligned.
Instead of just showing what the camera is pointed at, these imperfections could result in a bright multicolored ring, or even other shapes with the crazier lenses. You see it in movies filmed with real cameras, say when a camera pans past the sun, and with virtual cameras we've been aping it ever since. It looks cinematic! It looks like a real camera! Never mind that it's a (perhaps artful) optical mistake, we've been trained that this is what high quality images look like.
Both Syndicate and Mass Effect 3 are set in The Future, with a very specific look. Shiny surfaces, bright lights, and oh my christ are there lens flares. One could argue that with Syndicate being a first person game, in this lens flared future you're nominally looking through the eyes of the protagonist Kilo, so you shouldn't get lens flares consistent with an older 70mm lens, but he's had work done, man. Lay off. Robotic parts and shit. You don't know! Same goes for Commander Shepard in Mass Effect. Although, as a third person game, we view the action from a camera behind him. Who knew that during the reaper threat this cameraman would use the same camera setup as they did in Easy Rider.
J.J. Abrams went a little bit nuts with it in a similar fashion in his version of Star Trek. I'm not saying it doesn't look slick, because it does, but I think someday in our own shiny futures, this affectation is going to look more than a little ridiculous. John Carmack compared it, along with film grain, to adding horse shit to your car to get the authentic horse and buggy experience. As lenses and technology improve we'll be able to shoot the same scenes with less lens flare, not more. The problem is, if we're all wired to equate lens flare with The Future, we may not collectively want to.
Personally I want to see more of the things the camera is pointed at. I want more frames per second. Less lens flare. Even more dimensions if they ever figure out a way to do it right. I want more real in my pretend.
Art directors of the video game world, take note. Working together we can form a new high tech aesthetic. Join me, won't you?