The reason for country specific domains, such as
.fr, .de, .co.uk, .ca , .jp, .us, .dk, .be ... is so that people can register domain names and target their audience. Companies based in France will most likely have a .fr extension and their website will primarily be written in French, with an English link. This would also help the company to get listed in French directories and French search engines.
People do not always type in English words to perform searches.
If they search only using Google.com, then the .com and .net domains will be the dominant extensions. If you go to Google.de. Type in Frei pornographisch pics . You will see the domains ending in .de catering to free porn pics in the German language.
A lot of companies will register a domain targeting their country, but will also try and register a domain in the popular .com . People will then optimize their .com domain and sites and then submit them to English speaking search engines and directories. Especially since .com is a common type-in extension. It just allows for another way to draw traffic to your site, providing your site caters to English speaking people.
It's not that Google doesn't particularly care for domains ending in .biz, .info,... It has to do with popularity of the domain extensions. Most people stick with .com or country specific.
The .xxx extension and .sex have not been released yet. ICANN has not approved the top level domain yet.
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