When do you need to "alter database open resetlogs"?

[dba_admin]

This a recovery test. I dropped all control files and all datafiles while the db is up and running. Then, I use rman to do the following restore and recovery. As fas as I know, as long as you have all archived logs and online redo logs, you can recover and open the database without "resetlogs." Why does this one require me to "resetlogs"?

run {
allocate channel tape1 type 'sbt_tape';
allocate channel tape2 type 'sbt_tape';
restore controlfile;
alter database mount;
restore database;
recover database;
sql "ALTER DATABASE OPEN";
release channel tape1;
release channel tape2;
}