When to use Models ? (just need a little briefing)

[eluser]sikko[/eluser]
Hi all,

I've watched the two video tutorials but they don't explain when should we use models...
In the blog application for instance ? why don't even we use models ? Aren't they needed ?

If you take a look at [email=http://video.derekallard.com/]this sample application[/email], you will notice that the guy uses a model with only one function:

Code:

function getSearchResults ($function_name, $description = TRUE){
//the function...
}

Why haven't he put this "getSearchResults" function in the controller ?

Thank you for your help.
//CI Rocks!