Several reasons. If you want to optimize for flow (which makes sense if you want to shorten time to market), it makes sense to have <100% individual utilization. Maybe 80–85%.
If you are interested in understanding this, here’s a resource about Lean (which influenced the Agile Manifesto authors):
Lean Thinking - Large Scale Scrum (LeSS)
Does your organization measure "productivity" or "efficiency" in terms of how busy people are, or how much time is spent…
Watch the baton, not the runner.