For most jobs, if you've worked in the industry for 20 years, you have seen 'secret sause' of several companies. They often look remarcably similar. Indeed thats kind of the value of an experienced hire?
By only allowing people who don't know anything secret to work on a product, you keep the product safe from allegations (justified or not) of illegally incorporating third party material.
If you don't do it, you risk that a competitor claims "your product contains our proprietary material, pay us royalties or we'll sue your customers". Even if you're 100% in the right, until you've had that decided in court, your customers have to worry about getting sued, and you'll lose business (and also spend a lot of money on lawyers).
The model is particularly known/used in reverse engineering: One team (that is not tainted with knowing any NDA'd materials, I assume) looks at the product you're trying to reverse engineer, and writes a specification. Another team (not tainted by NDA'd knowledge or knowldege of the code of the original product) then implements the spec. https://en.wikipedia.org/wiki/Clean_room_design
How about security secrets which directly impacts the customer?
back in the day you would be tainted if you had read any of this material. today those lines have been blurred and even my employer, who was known to viciously keep tainted people out of the kernel, is nowhere near as strict