My impression from the slides is that they see it as a financial gain if it frees up the job for a higher performer.

No mention is made of why they feel the need to offer severance when they're shedding an underperformer, but maybe they want to make it a less negative event. Maybe the strategy is to try and avoid distracting or upsetting the high performers by having underachievers leave on good terms (this could explain the "goodbye" emails).