It's conceivable that a superior and a subordinate could have a mutual relationship. Yes, there could certainly be obvious cases where the advances are not wanted or reciprocated, but I'm not convinced that having uninvolved third parties filing complaints is the best route.

It certainly is conceivable that a superior and subordinate have a mutual relationship, but even in that case, as soon as they start a relationship they need to be separated from the direct report relationship.

That said, what I'm getting at, is that there are times when people step over the line into harassment in full view of others.

Not just in sexual harassment either. Verbally or physically abusive behavior falls into this category as well. Sometimes the victim may be afraid or unwilling to stick up for themselves but ultimately harassment hurts the whole workplace so third party reporting can be an extremely valuable measure to address the issue.