Fakeroot runs a command in an environment were it appears to have root privileges for file manipulation, by setting LD_PRELOAD to a library with alternative versions of getuid(), stat(), etc. This is useful for allowing users to create archives (tar, ar, .deb .rpm etc.) with files in them with root permissions/ownership. Without fakeroot one would have to have root privileges to create the constituent files of the archives with the correct permissions and ownership, and then pack them up, or one would have to construct the archives directly, without using the archiver.
