There are a lot of metrics companies can use. Browser cookies will tell a company if you clicked on an ad, and whether or not you bought something as a result. Another tool is discount codes; give one code to one advertiser and a different code to another, and see which gets used more. You could do something similar with phone numbers or websites.

Maybe because the current government of Germany isn't Nazi? If you moved into a house and 7 years later I sued you because the last tenant was an asshole, that wouldn't make much sense.