What is the significance of health in bt? What's the use?

The redundancy that peers you can connect to can provide together.

For example, if your health is 1000%, you can still download it even if 910 people leave.

If the health is just 100%, it means that there is only one seed, and he can't get down if he leaves.

If it is less than 100%, it means that a complete document cannot be pieced together and probably cannot be completed.