Pre aim(keyboard pre aim ,using A and D to line up crosshair)

pre aim with mouse

hug the wall

blablabla...

can someone explain me when and WHY to use a specific type of crosshair placement/ aiming style?

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cause for example sometimes i try to hug the wall with my crosshair, and when the enemy appears my crosshair its so glued to the corner that they past away from my xhair and kills me xD just an example