When you run MySQL commands MySQL, mysqlcheck, mysqdump and psql, psqldump, etc; they will pick username & password from this file if you do not provide them as argument (-u and -p). It can save you time.
Of course, if you specify username and password explicitly as part of the command’s arguments, they will be used.
.my.cnf ( for MySQL client )
[clienthost1] # Note: client + host1 user=myuser password=mypass database=dbname host=server.location.com