In these older books, one can see older models and really see how the game has developed over the years. Stuff like this really helps me decide how to paint an army. Seeing the old Ultramarines from the third edition codex is what helped me decide on the color scheme that I'm using now. I like it a whole lot better than the newer scheme with black weapons and gold everywhere. These older codices also have "model showcases" that aren't in the newer books. In the old Codex: Chaos Space Marines, I found a few converted minis that I'd never seen before. Codex: Space Wolves has a really nicely painted bike squad just inside the front cover. Codex: Dark Angels has a fantastic chapel diorama just inside the inside cover of that book. While the models aren't as good as the ones from today, there are some fantastic ideas and great ways to get inspiration for the hobby that just can't be found in the newer codices.
Also, the older codices are full of stories and background material that isn't in the newer versions of the game. This is my favorite part. The fluff that comes from these older codices sometimes doesn't match up completely with what's currently in print, but it clearly shows the evolution of the universe and of the game itself. In some of the xenos codices I picked up, I read a few stories of Imperial scientists researching things that, in the current editions, are known fact. And if you look closely, you may even read something that you never knew before.
To me, older codices add something to the game that I can't get from the newer editions. If I ever come across an old book that I don't have and isn't in print anymore, I'm probably going to buy it on the spot. If you want to get heavily into the Warhammer 40,000 background then I suggest you keep an eye out too. They're hard to find, but well worth it.