What is the primary purpose for chamfering concrete elements. Is it for architectural or safety reasons? Is it forthe ease of stripping the formwork?

Both, architectural and safety. it recommended to use triangular plastic strip in form work corners 

for foundation (buried) elements, chamfering makes the correct application of insulating membranes much easier. for fair-faced concrete, you avoid sharp edges which can easily break and also mitigates surface cracking propagating from the edges.

to stop the corners of concrete breaking off. otherwise you would have a 'sharp' corner which is difficult to maintain

I would accept the standard practice but not without questioning the rationale. Usually, concrete members will be buried in block work and the chamfers are often buried in mortar.

As for the safety concerns, I see it from a different perspective. On many occasions, the final architectural details require "unchamfered" edges. Meaning that any chamfered edges will be "unchamfered". If it was a safety concern, an architect would not allow such corners in children school buildings for instance.