Why is causation important in negligence cases?

The burden of proof regarding causation is on the plantiff. Causation is important because - sometimes - a person may have died in a hospital (for example) for reasons other than negligence (for example, a pre-existing condition rather than actual negligence...and negligence must be proven).

Typically, an expert witness will show causation (or an act of negligence) for the plantiff, but the defense counsel will rigorously oppose any plantiff expert witness.

The litmus test for neglegence is usually permanent harm or death. Otherwise, the odds are lessened in a personal injury suit.