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What is so great about Unix sockets that e.g. a normal TCP socket bound to localhost or even a named pipe can not do properly?

UNIX Sockets are more flexible than named pipes;

* You can use them for more than two processes communicating (eg. a server process with potentially multiple client processes connecting);

* They are bidirectional;

* They support passing kernel-verified UID / GID credentials between processes;

* They support passing file descriptors between processes;

* They support packet and sequenced packet modes.

TCP only grants you 2 of these extra features (sequenced packet mode/bidirection), leaving a giant hole in security in the process.

The named pipes, at least on Windows, also support all of that except for passing UID/GID and file handles.

The server can get the calling process info [0], which may be useful for local access control. TCP sockets cannot distinguish clients like that.