The essence of steel in a reinforced concrete structure, such as a beam, can be summarized in the following key points:

1. Tensile Strength: Concrete is very strong in compression but weak in tension. Steel reinforcement is used in concrete structures to resist tensile stresses. When a beam is subjected to load, the top part of the beam experiences compression while the bottom part experiences tension. By placing steel reinforcement in the tension zone, the beam's tensile strength is significantly increased.

2. Crack Control: The presence of steel reinforcement in concrete helps to control and limit the width and spread of cracks that may develop due to shrinkage or temperature variations. This improves the durability and longevity of the structure.

3. Structural Integrity: Steel reinforcement provides continuity and ductility to the structure, allowing it to withstand dynamic and seismic loads without collapsing suddenly. This is particularly important in earthquake-prone areas.

4. Shape Flexibility: Steel reinforcement can be bent and shaped as needed, allowing for the construction of complex structural shapes and forms that would not be possible with concrete alone.

5. Load Bearing Capacity: The combination of steel and concrete in a reinforced concrete structure results in a material that can bear heavy loads. The compressive strength of the concrete, combined with the tensile strength of the steel, allows the structure to support significant weight without failing.

6. Fire Resistance: While steel alone can warp and lose strength when exposed to high temperatures, encasing steel within concrete provides a degree of fire resistance. The concrete acts as a barrier, slowing the rate at which heat reaches the steel.

In summary, the use of steel in reinforced concrete structures enhances their strength, durability, flexibility, and safety.