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.
.pgpass ( for psql client )
test1.net:5432:testdb1:testuser1:testpass1
test2.net:5432:testdb2:testuser2:testpass2