Yes it can be used when  u want system to point to a 
particular File/Member in a library while processing a file 
in a high level pgm. Example, suppose u want to use a work 
file in QTEMP , although other object of the same file 
exist in the current library list. other way to achieve 
this is by using Keyword EXTFILE and EXTMBR on the F spec 
of that file in RPGLE pgm.
I think OPNQRYF can be used without OVRDBF but that 
physical file should have been defined/changed with SHARE
(*YES). This will create a data path for the file to be 
shared.

OVRDBF can be used separately without opnqryf to override 
the attributes of file.
Now, OPNQRYF without ovrdbf, is also possible:
opnqryf creates the open data path and to share the same 
data path in a program we use ovrdbf share(*yes), if we do 
not use ovrdbf, then program will have its own ODP, so it 
will read the file from begining on which query has been 
run. So, OPNQRYF selected records will have no importance.
So, we need to use CPYFRMQRYF command and put the selected 
recoreds in to runtime file and use that file to read in 
program.