PMP is one of the clearest leading indicators a maintenance team has. World-class operations typically run above 80–85% planned work; a team stuck below 50% is spending most of its week firefighting, which leaves no capacity to actually get ahead of the failures causing the fires.
It's a leading indicator because it predicts next quarter's downtime, not this quarter's. A team with rising PMP is building slack into the schedule; a team with falling PMP is burning through it, even if this month's uptime numbers still look fine.