Well with lib you are saying that directory is the *first* place to look for modules, with push you are saying it is the last. Sometimes you might want that control.

I once had to deal with a Perl system that didn't know where its libraries were, manipulating @INC in a BEGIN block was what got me out of that. OTOH if you know about PERL5LIB (which I do now and didn't then) then that is easy enough to fix.

So yes, I can think of uses. But none that I would cause me to recommend manipulating @INC rather than using lib.