Why we are using top reinforcement in beams and sometimes in slabs (The benefits of using the top reinforcement)?

The reinforcement rebars must be provided in the top layers to resist the flexural stresses or bending stresses. 

Because based on the code provision and theory of structure in the design analysis the negative moment on the support which is possible bending is upward the reinforcement rebars must be provided in the top layers to resist the flexural stresses or bending stresses. 

because to resist the negative moments and reduce the depth of beam.

to take compression and reduce thickness

1 formwork setup2 placing of bottom reinforcement3 fixing of bar chair for post tension tendon4 placing post tension tendon5 fixing anchorge burstling reinfirement6 placing of top reinforcement7 placing anchorage pocket former and grout vent8 fixing slab edge formwork with pocket former9 concreting of slabs10 remove slab edge formwork

for many reasons,

1- shrinkage crack caused by high temperature resulted of chemical prosses for cement .

2- support to carry stirrups .

3- responsible about  resist little bit of negative moment near to supports .

There will be tension force on top half of the beam on some sections of continuous beams as well as on cantilever beams. So it is required to provide top reinforcement. Same rule is valid for slabs too. Top reinforcement aids in constructability.  And it provides the beam with some capacity to resist negative moments, such as those caused by earthquakes or some unprecedented loading condition.

for cantilever beam tension is at the top of beam so we need reinforcement.

now most of beams are doubly reinforced because it can carry more imposed load