Can someone help me understand the re-releases?

[Mobjack]

I was looking through the Upcoming Releases pages to see what movies were coming soon. I noticed a fair amount of titles that were being re-released. Many looked like they were packaged differently and others had little promos with them. I can understand with movies like Gladiator, that had problems and needed a re-release. My question is, with all of the great movies out there that have yet to be converted to blu, why are companies spending time and money to re-release titles that are already out there?

Thanks!