The hook is provided in stirrups for the following reasons
1- To prevent buckling of the column.
2- The main requirement for safety against bond failure is it
provide a sufficient extension of the length of the bar beyond the point where the steel is required to develop its yield stress and this length must be at least equal to its development length. However, if the actual available length is inadequate for full development, special anchorages must be provided, such as cogs or hooks, or mechanical endplates.
3- Hooks are provided to resist seismic movement.
4- To prevent concrete from splitting outward.
5- It prevents slippage of steel from the concrete.
6- To Keep longitudinal steel bars in position and hold steel tightly.