why is this code here?

["Sean 'Shaleh' Perry" <shaleh@valinux.com>]

in dbootstrap/choose_medium.c:choose_NFS_server()

for (;;) {
    /* keep user entry between calls to choose_NFS_server */

    if (!preventry) preventry=strdup(""); <<-- why??????

    nfsmountpath=inputBox(_("Please choose the NFS server and the mount path of
the NFS filesystem that contains the Debian archive.\nEnter them in this way: 
server:/ftp/debian"),_("Choose Debian NFS filesystem"),preventry);
    if (! nfsmountpath) return 1;
    free(preventry);
    preventry=nfsmountpath;

.....
}

preventry is set to NULL at the top, strdup("") just sets it to NULL again. 
What am I missing????