What problem do MOM (Message Oriented Middleware) solve? Scalability? Integration?

In which domain are they typically used and in which domains are they typically not used?

For example, say, is Google using such solution for it's main search engine or to power GMail?

What about big websites like Walmart, eBay, FedEx (pretty much a Java shop) and buy.com (pretty much an MS shop)? Does MOM solve a need there?

Does it make any sense when you're writing a Webapp where you control the server-side and have an homogenous environment (say tens of Amazon EC2 instances all running Linux + Java JVMs) there and where the clients are, well, Web browsers?

Does it make sense for desktop apps that need to communicate with a server?

Or is it 'only' for big enterprise stuff where you typically have a happy mix of countless of different systems that needs to communicate in a way or another?

I'm a bit confused as to what they're useful for and I think that with example of where they're appropriate and where they're not appropriate I could better understand their use.