Why do you need to manually set base_url in config ?

[eluser]bernstable[/eluser]
Been puzzling over this and I must confess to not being able to understand why you need to manually set base_url in the config file. Surely the following ensures that the base url is set correctly whether viewing locally, remotely or if you have moved your application to a brand new and shiny url

$config['base_url'] = 'http://'.$_SERVER['HTTP_HOST'];

And with my limited knowledge I'd have thought that the above would be the most logical default setting in the CI files as it catches all.

Am I totally missing the point? Can anyone tell me why I shouldn't be doing that? (Go easy on me I've only just started with CI but I'm loving it)