You usually won’t notice much difference, but both the blur effect and the bloom script are not similar to the glare shader. Glare uses a higher luminance range to calculate the glows/glares while the blur effect and bloom script use the standard 0-255 RGB space.
This can be seen when you have an super-bright object; this will always be displayed as pure white (255,255,255). The problem is that the range ends at pure white so there is no difference between super-bright or bright: it’s all white to your program. Higher range spaces can overcome this problem by storing colour information beyond 255 and below 0.
What the glare shader does is it takes that colour information beyond the 255 range and scales that down to make the glow.
Example, say you have a yellow object (R:125,G:125,B:62) and the object starts to get brighter and brighter (say a white light starts to shine on it).
With a low range colour space the RGB levels first go to (R:255,G:255,B:125), here they get ‘stuck’. As the object gets brighter the levels go to (R:255,G:255,B:255), the object now appears to be pure white. When a glow is applied based on this range you would get a white glow around the object. This is far from realistic as the glow is not the scaled down bright yellow colour.
With a high range colour space the RGB levels first go to (R:255,G:255,B:125) (compared to low range RBG), here they can continue on. As the object gets brighter the levels go to, say, (R:1000,G:1000,B:500), the object still appears to be pure white. But when a glow is applied based on this range you could get a yellowish glow around the white object. This is the most realistic of the two.
Of course the appearance of a coloured glow depends on the brightness of the object and when the objects start to ‘glow’.
Another advantage of the glare shader is having only the brightest of brightest of object emitting a glow. With the blur or glow effect all saturated parts of the image will glow. With the glare shader some white parts might glow while other white parts might not.
For post glows, like in Photoshop, this is also the case. The glares/glows can be much more realistic if you use HDR or OpenEXR images.
Super-bright objects only look that bright in photographs because of the glow around them. If the camera would have a perfect lens then the object would appear to be just as bright as a regular bright object on the same photograph.
The glow comes from internal reflections inside the camera lens. In the lens the light loses a lot of its energy because of the internal reflections, this is why you can sometimes see the base colour of an object that appears to be pure white.
Glows can also be caused by light that passes through the film, hits the backplate and goes back into the film again (halation). But that only happens with IR film or film that saw dinosaurs walk as modern regular film has a special layer to prevent this.
The internal lens reflections have several different causes. The geometric shapes seen in lens flare are caused by the diaphragm, so are the streaks/spikes (there are twice as much streaks as diaphragm blades if I’m not mistaken). The ghosts are caused by the different lenses inside the camera lens.
Dirt and scratches on the lens can also cause light scattering. This always happens with lens imperfections but it only gets noticable with very bright light.
The same goes for your eye really, it also has a non-perfect lens.
Pff, enough technical stuff for one day, time for beer!
Hope this helps!