For security purposes – If you call back a machine, you can be sure it is the right machine.
The string CALLBACK=yes specifies that your computer must call back the remote computer before any file transfers can occur.
The default for the CALLBACK option is CALLBACK=no. If you set CALLBACK to yes, the permissions that affect the rest of the conversation must be specified in the MACHINE entry that corresponds to the caller. Do not specify these permissions in the LOGNAME, or in the LOGNAME entry that the remote machine might have set for your host.