The person doing the specification has been "contaminated" by the original's documentation and product; it's possible to argue that they carried over knowledge

The person doing the specification has been "contaminated" by the original's documentation and product; it's possible to argue that they carried over knowledge directly from the original product. The lawyer in the middle establishes a firewall ensuring nothing was taken from the original outside of the specification. I don't know of any actual cases that hinged on this, but lawyers tend to be belt-and-suspenders types. One remnant is the advice from the GNU folks on building clones of Unix tools---you could even be tainted by looking at the Unix source, as long as the clone was architecturally very different. (Which led to a lot of better implementations of Unix tools...)