Use case: Devel::Trepan currently reports offsets when it is stopped. It gets this from Devel::Callstack currently. However in the past it didn't, although I can't remember the details, other than it was by some other means as an undocumented feature (which means it is not guarenteed to work in the future).

More generally, if the full OP were available, more information could be gleaned about where Perl is at any given time. It also might be used as a way in decompilation to walk around the point of interest rather than starting from the top and decompiling everything.

I'll be giving a talk on this at Glasgow YAPC 2018, so come see me there for more information. ;-)