why DBD::mysql need database name for connecttion ?

in the pod:

####################
   Class Methods
       connect
               use DBI;

               $dsn = "DBI:mysql:$database";
               $dsn = "DBI:mysql:database=$database;host=$hostname";
               $dsn = "DBI:mysql:database=$database;host=$hostname;port=$port";

               $dbh = DBI->connect($dsn, $user, $password);

           A "database" must always be specified.

###################

why database is needed ?
the mysqlclient can connect to server without this, and then select database.