Why does tempuri.org exist? Why does each XML Webservice require its own namespace, unique from any other on the web?
Plug http://tempuri.org/ into your browser, you get a fairly good explanation there.
Updated sadly the tempuri.org
URL no longer works but the domain is still registered with Microsoft, maybe one day it'll come back.
It's also worth reading section 'A 1.3 Generating URIs' at:
http://en.wikipedia.org/wiki/Tempuri
tempuri.org is the default namespace URI used by Microsoft development products, like Visual Studio.
Probably to guarantee that public webservices will be unique.
It always makes me think of delicious deep fried treats...
Webservices require unique namespaces so they don't confuse each others schemas and whatever with each other. A URI (domain, subdomain, subsubdomain, etc) is a clever identifier as it's "guaranteed" to be unique, and in most circumstances you've already got one.
Note that namespaces that are in the format of a valid Web URL don't necessarily need to be dereferenced i.e. you don't need to serve actual content at that URL. All that matters is that the namespace is globally unique.