NIST 800-90C defines 3 classes of random bit generation (RBG) constructions: RBG1, RBG2 and RBG3. All constructions must include a DRBG from NIST SP 800-90A. The particular construction in question is RBG3, which is "designed to provide output with a security strength equal to the requested length of its output by producing outputs that have full entropy" and is based on a physical entropy source. This construction has 2 types, RBG(XOR) and RBG(RS).
I am going to guess that "full entropy" is defined as entropy per bit above some threshold like 1-2^32 or 1-2^64, not sure what the actual threshold is here so please correct me.
Why does this recommendation require a RBG3 construction based on some physical entropy source to use a DRBG that is XOR'd or re-seeded by the entropy source to produce full entropy?