Measuring web site use

One thing is not clear to me.
To calculate the price of a space for an ad, both TV and radio, have estimations of the audience that could be exposed during specific programs and times in the day. If you want an ad you just have to multiply the cost of the space you want for the times you want to run your ad. So I guess works for the Internet. Do you want to place an ad? Just multiply the time your ad will run for the price assigned on the base of the estimation of the amount of audience that will see that site.
However, the New York Times article says that "[the] audience measures are not used to set Internet advertising rates, as they are in radio and television [...] On the Internet, Web sites charge to display an advertisement a certain numbers of times."
I'm confused.
So what's the purpose of audience measuring? Why do it? Why media buyer are asking for more precise estimations? Why they are asking, according to NYT, for estimation models that resembles those used for traditional media?
If audience measures are not used to set advertising rates, how are calculated ad spaces on different sites? An ad on the NYT is more expensive than one, let's say, on my site...