Give reason why the health and safety policy should be signed by the most senior person in an organization.

Top Management commitment is the key in any organization success, in order to maintained good health and safety senior personal from the organization should acknowledge the health & safety policy and make him self committed to provide resources, follow up on outstanding health and safety projects and obviously be accountable for any discrepancies.

To confirm the commitment by the top management authentication of the policy.

Policy should not be denied by subordinates. It considered as the vision and mission for the business strategy to follow.

it shows commitment from senior management in implementing health and safety through training and funding. it shows dedication and leadership in promoting health and safety. In a nut shell if the boss can do it everybody can do it

It is important that the safety policy to be signed from the most senior in the organization to ensure the commitment and the buy-in of the safety policy from the top management through all the levels of the organization , , ensuring that safety goals will be supported...

in order to create a culture that foster safety and aims to acheive a high levels of safety standards , involvment of top management and senir  is a must...
this mission is joint effort , otherwise , if it is restricted to one department to hold the responisbility to do the required changes , the efforts will be futile .
all the time will be a be a gap between the aimed goals and the actual results on the ground...

It's the Committment that are required from the Top before communicating it to the bottom.

because the commitment of senior mangement is the success key. If they were committed, junior level employees will follow that commitment and take the next step to improve.