- people will be more willing to talk to investigators if they know that their evidence won't be used against them in court or used for a licence suspension
- you can grant non-regulatory investigators greater powers to compel testimony and documentary evidence, perhaps even if it's self-incriminating (I can't speak for the US but it's common here in Australia where we don't have a 5th Amendment entrenched guarantee), without running into as many ethical issues as when you give coercive information-gathering powers to regulators - you ban such compelled testimony from being used as evidence in court or in regulatory proceedings
- you don't need as high a standard of proof to make an investigatory finding as if you're taking punitive action against a specific individual
- the overall purpose of the investigation is aimed at systemic safety, rather than getting a successful prosecution
None of this prevents the regulators from running a parallel investigation with the aim of regulatory punishment - and indeed, two sets of independent eyes on complex scenarios is good too.