historically, Complex a
didn't have a storable instance in base,
and thus storable-complex was born!
you're using an older ghc/version of base AND calling out to some C or fortran codes around complex numbers. You might find yourself needing this then!
if you're writing your own orphan instances instead of using base-orphans, you might have a bad day, use base-orphans (and storable-complex) instead!