One of the main functions of visual perception is to detect objects in the environment as either a potential mate, predator or perhaps lunch (Scientific American p18. V Ramachandran, D Rogers-Ramachandran). So determining what is not part of the scenery has been vital in the past in order for us to pass on our genes to our lucky predecessors. What use is it in modern life though? I'm not in the habit of popping out to the high street to bag a mammoth or flee from roaming tyrannosaurus rexs (!) on my way home from work.
What's the use of it now in getting our food or our mate then? Well in design terms it means that when we engage with visual communication we tend to see contrasted items much better. It jumps out at us as we say to ourselves, "Woah...is that a dinosaur?!". Use your head, designers!
No point using low contrast text on designs...
When I was doing my O levels the dreaded exam papers were black text on yellow paper. Oh boy did that strike fear in our hearts. We only had to glimpse a registration plate to give us the shivers! Apparently though this is the best contrast for legibility. So I guess we should thank the exam board for making it like this!
If you need some tools to ensure your website text has sufficient contrast for good legibility check out this article which gives some web tools to use.