RMAN-08137: WARNING: archive log not deleted as it is still needed

the warning messege appear in rman log file After do rman backup with the following commands :===============================================================================
RMAN> run {
2> allocate channel c1 type disk;
3> allocate channel c2 type disk;
4> backup as compressed backupset
5> format '/u06/import/rmanbkp/%d_%u.bkp'
6> database;
7> sql "alter system switch logfile";
8> backup as compressed backupset
9> format '/u06/import/rmanbkp/%d_%u.alg'
10> archivelog all delete input;
11> sql "alter database backup controlfile to ''/u06/import/rmanbkp/control01.ctl''";
12> }
13>
=====================================================================
How this warning mean :
                RMAN-08137: WARNING: archive log not deleted as it is still needed

And how solve this problem.

best regards

Magdy Mohanna
IT Manager