In this instance, it appears the author's motivation was to facilitate a clean room reimplementation by "producing a natural-language specification for others to reimplement". In other instances security researchers might reverse firmware in order to find vulnerabilities. As the article states:
> One example motivating the production of open source firmware for the BCM5719 is that it's the only closed-source firmware blob found in the Talos II, a high-performance POWER9-based system otherwise wholly free of firmware blobs... Once this is delivered, it will be possible to use Raptor's POWER9 systems with purely 100% free, open source firmware. As far as I am aware, there is no other machine in the same performance class which can make such a claim.
> One example motivating the production of open source firmware for the BCM5719 is that it's the only closed-source firmware blob found in the Talos II, a high-performance POWER9-based system otherwise wholly free of firmware blobs... Once this is delivered, it will be possible to use Raptor's POWER9 systems with purely 100% free, open source firmware. As far as I am aware, there is no other machine in the same performance class which can make such a claim.