A questions about SP 800-90A DRBG additional input There is four inputs in SP 800-90A DRBG that is entropy sources, nonce, personailisation string, additional input.

I noticed the soundness of entropy_sources is most important thing for DRBG security and then nonce and last is  personalisation string.

I am wonder why additional input is needed? because entropy source is the key for security.

Here is my question


please give me an example of additional input for software cryptographic library (for linux, windows, anything..)
Is additional input is crucial to DRBG security?
Some (hopefully) useful information to what you ask can be found in sections 7.2 Other Inputs and section 8.7.2 Additional Input of the SP 800-90A Rev. 1   June 2015   Recommendation for Random Number Generation Using Deterministic Random Bit Generators standard at http://dx.doi.org/10.6028/NIST.SP.800-90Ar1.

I copied and pasted a couple of points which I hope will shed some light, but I would recommend reading those sections in full.

7.2:
Other information may be obtained by a DRBG mechanism as input. This information may or may not be required to be kept secret by a consuming application; however, the security of the DRBG itself does not rely on the secrecy of this information.
...
Additional input may also be provided during reseeding and when pseudorandom bits are requested. See Section 8.7.2 for a discussion of this input.

8.7.2:
Additional input may optionally be provided to the reseed and generate functions during requests. The additional input may be obtained from inside or outside a cryptographic module, and may include secret or public information. Note that a DRBG does not rely on additional input to provide entropy, even though entropy could be provided in the additional input, and
knowledge of the additional input by an adversary does not degrade the security strength of a DRBG.

I'm not sure of real world examples.