WHY CRANK BARS ARE PROVIDED IN SLAB ?

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Cranked bars / Bent up bars are provided on top of the reinforcement to resist negative bending moment/Hogging at the supports of the slab.
To resist the shear force at the supports.
To reduce the risk of a brittle failure of slab-column connection.
When bent up bars are provided, the strength and deformation capacity of slabs with bent up bars compared to slabs without bent up bars is sufficiently increased.

And Mainly ,

( 1 ) .Bottom Reinforcement resists the positive bending moment/sagging at the mid-span of the slab.Top Reinforcement resists the negative bending moment at the supports of the slab.

( 2 ) .It is actually used for the economization of material. Normally, for fixed ended Slabs positive (sagging) moments are present at the mid span and negative (hogging) moments at the supports.

Note :- Bars are bent near the supports normally at an angle of 45°. The angle bent may also be 30° in shallow beams where the (effective depth < 1.5 breadth).