The results of “Progress Override” are not realistic, then why this function is there in Oracle P6?

Progress override has to do with statusing. If a successor starts before its predecessors finish, there are two choices. One is Progress Override, which means that decisions made in the field will be observed even if they violate the schedule logic. For instance, Construction starts building before the Design is done, thinking that he knows at least some of the construction and does not have to wait until the design is totally done. But management may understand that the design still has some unresolved issues that will change construction sufficiently. The successor may have made several days or weeks of progress, but is told to stop and the remaining work waits for the design to finish. Basically, while retained logic and progress override are two general approaches available in P6, the scheduler needs to examine the issue case-by-case to see which is appropriate. I would not rely on either general solution in all circumstances.