By definition, a perimeter is the outer boundary, and securing that perimeter means protecting that area. Everything has a perimeter, from a large university campus to your own personal space on that campus. Law enforcement and security personnel have predefined outer perimeters that keep people either in or out in the case of an event. But you should also look within the perimeter to criticalities that could present problems.
Physical protection of electrical substations is much more critical than simply guarding against copper theft. Recent terrorist attacks on substations and the potential for simultaneous attacks across the U.S. put the entire country at serious risk for major power interruption.
The FAA projects a 30x growth of commercial drone use to 600,000 over the next year, which could pose a significant threat to physical security and critical infrastructure.