The violet colored laser can be made thinner than a red one, allowing the physical data spiral to be smaller, leading to greater data density. This allows the disc to hold substantially more data than a C.D. or a D.V.D. More data means you can get away with less compression, leading to a better picture quality and store more content.
Aside from that, even just rebranding optical discs would allow device manufacturers to update their software specifications, without confusing the consumer into thinking new discs would work on older players. This allows for newer codecs/filetypes, which are more efficient and less constrained. MPEG 2 for instance, has limited colorspace compared to H.264. It also allows us to take advantage of newer hardware for tasks which would've been considered overly resource intensive and/or pointless in the past, such as outputting higher resolution video.
The end result is more detailed and lushly colored imagery with fewer compression artefacts from lossy compression schemes. You don't have to take my word for it though, see it for yourself:This is a comparison of Ghibili's Blu-Ray edition of My Neighbor Totaro next to a DVD counterpart
. If you look closely, you'll see the DVD doesn't have the resolution to detect the film's original grain texture and its colors look quite white-washed out in comparison.
To me the blu-ray looks much better. It seems almost like a painting, which makes sense since it's essentially a photograph of one.Edited by Tonepoet - 5/16/13 at 9:45am