Heart rate variability and cardio-respiratory synchronization are only measurable using relatively recent modern technologies, but both are highly correlated with positive mental health-related states, including a sense of general well-being and resilience (especially in traumatic and stressful circumstances).
These are metrics, in other words, that can help us measure someone's general affect and psychological state. And that's valuable because we don't have many other ways to do this, and most of the other options (like self-reporting and brain-scans) are either prone to bias or not yet definitively associated with a person's subjective state of mind (brain regions might light up in recognizable patterns, but those patterns can mean very different things from person to person and be associated with several different things).
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