It is quite interesting .
so if you get a 1000 visitors a month , rank no for keyword which gives 15 000 hits a month .
You ending up with 500 unique visitors a month from your links .
Then if you go ven further with sort of affiliate marketing or click tracking you can see which links give you the extra hits.
Quite nice.
Im sure lots of the big guys use a tracking software to determine this already.