Evidentially is a formal adverb about evidence, not about what merely “seems obvious.” When you say something is evidentially weak, important, or relevant, you mean: if we look at the proof we actually have — or at what a court could use as proof — that is how it stands. Lawyers, judges, and academics use it far more than ordinary speakers. “Evidentially, the witness helps the defence” means “as evidence, that is what the witness does,” not “anyone can see that.” People sometimes stretch it just to sound careful and official, when they really mean “from what we can see.” Though actually, it sits next to a much commoner cousin, evidently, which means “clearly / as you can easily tell.” Those two are easy to mix up, and only evidentially is about proof.

證據上；就證據而言（法律、學術用語；勿與 evidently「顯然」混用）