Tyre Wear ?

[chillerman]

Why is it that performance tyre treads over the last few years have started to use varying depths of tread over the whole tyre. I have noticed that when new you can see deep (8mm?) grooves on the main tread blocks but shallower grooves in various designs that intersect with the main blocks. These often wear and disappear early in the tyres life whilst the tyre still has essentially at least half of its life left. Wouldn't this change the tyres performance characteristic, particularly in the wet ?

I'm guessing it doesn't affect it to any great degree and may be a cosmetic thing but I'm not sure. Any comments/insight ?