WP 2011 template/sidebar stucture confusion.

Please can someone explain to me how the WP templates work, they don't make sense to me. I am using 2011 theme.

Here is what I mean.
When you create a page you have a choice here of templates to select.
Default, showcase or sidebar. Like the screen shot:
(please ignore the vanilla template, that is not used)

Then when looking at the widgets set up you can select which widgets you want for each sidebar. Like the screen shot:

You have showcase sidebar - which by default go to the sidebar page. And main sidebar which goes to either the sidebar or default template the same.

So the question is ; What the heck is the difference between default template and sidebar template. They don't make a damn bit of difference to the page. The showcase is the only one that invokes a page layout change. So what then is the point of having the two?

Even deploying a custom sidebar plugin doesn't use the template structure any differently. Sorry but I just do not get it.

Can anyone enlighten me please?