There are projects which provide daemons needing to be able to create, modify and delete unix users. or just switch user ids to interact with the system, for example a user space file server. to be able to test that you need the privilege to modify the passwd and groups file. with nss_wrapper it is possible to define your own passwd and groups file which will be used by software to act correctly while under test.

if you have a client and server under test they normally use functions to resolve network names to addresses (dns) or vice versa. the nss_wrappers allow you to create a hosts file to setup name resolution for the addresses you use with socket_wrapper.