In agreement with Intruder, it's not law (yet), but operator's SOPs.
Essentially it's all part of the "See and be Seen" process. No real need in Class 'A' fully controlled airspace, and even there, traffic does become congested below 10,000 feet, or below the Transition Level, whichever is higher.
Also it's common practice in FIRs where communications are very unreliable, e.g. Afghanistan, Yangon FIRs etc.
Regards,
Old Smokey