I don't think it makes a difference unless the domain itself contains special (multibyte) characters, such as the ones recently approved by ICANN.
You can have a page with a .cn domain that is all in English, and it will turn up in English results. Additionally, I've seen lots of .us domains in Russian, Chinese, Spanish, etc.
What matters is the locale used when publishing, and vanity. If an e-commerce shop boasts "Located right in the heart of the UK!", then it would probably want a .co.uk domain. Then again, most US businesses would want .com domains, rather than .us.
The other use is organizing your network. Servers in the US might be servername.foo.us, where servers in China might be servername.foo.cn, especially if redirection based on geo location is in use.