Opinions will differ on this highly subjective question.
> How does this fact aid my understanding of the severity of what he’s done?
It aids your understanding of the potential consequences, not the severity of what he has done.
In this case, probably not - 20 years is pretty stiff, and the crime implies that typical sentences are much less than that.
But if the maximum sentence was say... 6 months instead; or just a fine. Yeah - I think that would be useful information.