Libsmpw is a drop-in replacement for getpwuid()/getpwnam(). Like many versions of these routines it uses external index files for faster access. Unlike other versions, the index files are "self-maintained". This means they are world-writable, and every time any user runs a program that accesses these routines, the indexes get updated as necessary.
This is mainly to avoid the need for root privs to get fast /etc/passwd access - although even sysadmins may wish to install these routines to avoid maintenance headaches and delays in password changing.
This package is of somewhat limited usefulness. The main conditions under which it's appropriate are a system with: lots of users, no out-of-the-box indexed passwd access, and butthead sysadmins.