You are correct that the regulators are generally concerned with safety and efficacy of the final product, not the sausage making. But if the sausage making is super complex and/or novel that could potentially have an impact on safety and efficacy. So the company may have been asked to give some amount of detail there. For example: given this vaccine requires cold temperatures or it breaks down, how are they guaranteeing the cold chain in the manufacturing process? (I'm just using this as an illustration of the flavor of question someone on the committee might have)
Given how new this vaccine tech is, there are probably quite a few manufacturing-related efficiencies and techniques that Pfizer does not want competitors getting access to. Not because they're going to copy this specific vaccine, but because it allows them accelerate efforts in this area for other vaccines.
And ultimately make the world a better place? I fail to see any downsides here.
When a new car engine is developed you don't need to share exactly how you make the parts or how it works (of course knowing how it works for a vaccine is probably important), it just has to meet certain standards regarding efficiency and emissions to be approved by regulators.