https://stackoverflow.com/questions/3715925/localhost-vs-127-0-0-1 Basically its a socket performance reason.
also backed by: http://stackoverflow.com/questions/19712307/mysql-localhost-127-0-0-1
"As you can see here, a UNIX mysqld uses sockets if used without a host name or with the host name localhost.
So it makes a difference, and in the GRANT system this difference becomes evident."