crank is used to offset the bar form the direction. when we lap two reinforcement bars in column, if we lap them in side then cover will increase. therefore we crank the reinforcement bar and take it in to the same location where the bottom bar was. then the cover to the reinforcement will not change.
that why we do crank
there are other reasons.
if there is a level drop in a slab and if we want to keep the continuity then we crank the slab reinforcement.