One of the nice enhancements in z/OS Release 13 JES2 was the support for SYSIN in a JCL procedure. (See here for the announcement letter.) I have a personal example of where it would've been handy. You probably have your own. We used to distribute sample JCL to use DB2 DSNTIAUL to unload the DB2 Catalog, one table at a time. Obviously with such a repetitive use in a single job step you'd want...
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jcl
dsntiaul
z/os
db2