Is it necessary to stamp the approved shop drawings as AFC "Approved for construction" and why?

any approved drawing not stamped for approval to be executed is not safe to contruct because there is not legal evidence that this can be. that way everyone can throw a drawing on the floor and then you wouldnt be able to know which one is the approved one.

Yes it is necessary.

This confirms that the proposed layout is confirmed for construction by way of design and details. This allows for contractors to begin work without compensating for major changes.

Any approved shop drawings shall be stamped by the engineering/technical manager as AFC and supersede the previous revision prior issued to construction team. As a live example, if we got a clear approval on a shop drawing revision and then after while you received a new revision from the design drawing have some design modifications via an engineer instruction so, you should revise your shop drawing accordingly with a new revision then to resubmit for approval. Here is the trick, if you didn't stamp your shop drawing revision and supersede the old revision, the construction team will not be noticed that the revision has been superseded and revision got place then, the construction team will continue work on old revisions and will have a miss documents management lead to disasters.

Yes, as this is the basis on which the Contractor can proceed to work on site & prepare his shop drawing.