When I started noticing quite a lot of old/classic movies being released on Blu-ray, I figured it was a cash grab on the part of the movie studios. While you may end up buying something again that you already owned on VHS (yikes!) or DVD, if properly done an older movie Blu-ray can look great - often very noticeably better than DVD.
I guess my confusion arose from the assumption that Blu-ray is an HD format, and I reasoned that since HD is "new", then those old movies would just be digitally "fluffed up" to look a bit better on the new medium. Not necessarily; movies made on 35mm film are effectively at a higher resolution that HD TV today. If you have decent source prints, you can produce very attractive Blu-ray versions.
The "Wizard of Oz" was made in 1939 and was restored a few years ago for a DVD release. It has since be remastered for Blu-ray, and looks ever better. Now, this was a full-blown restoration, but other more recent films can be more readily put on Blu-ray with great results - movies like the Godfather, Blade Runner, and so on.
You really can see and enjoy older movies like never before - unless you saw them in the theater back in the day - with clean (often pristine) audio and video.