They use this data to produce an 'Alexa traffic ranking' of all sites. This can be seen on the Alexa search toolbar and is used by most smart web masters to determine how popular a site really is.
Alexa uses the largest sample data base in the world to determine site popularity, over 10 million, and so can be relied upon for accuracy, but I keep getting asked,
Answer. . . Certainly, up to a point, but why would you want to?
It is possible to fool Alexa into thinking that your site has an Alexa traffic ranking of 500,000 when it should have a true rank of over 1,000,000. In extreme cases, it might be possible to drag it down to 150,000.
This is usually done (with great difficulty) by hitting the Alexa toolbar from a large number of different ISPs scattered around the globe using rotating ISPs and cookie controls in an automated fashion.
If you are particularly paranoid about your Alexa ranking you can even pay some companies to do just this, but here's the real point.
Why spend all this time and money achieving a false rating, when you could more usefully achieve the same result by putting honest information onto your website which visitors would naturally want to read?
This will cause the search engines to want to list your pages favorably and so with the increase of free visitors, your search ranking will rise anyway and probably your sales also.
Isn't that the real point of having the site in the first place?