Adobe over the years have had a number of different ways of tracking channels and modelling attribution. This one I believe pre-dates marketing channels. There is no right or wrong answer as to when to use it (assuming you've got it set up correctly), it's just going to tell you something slightly different. However in our organisation we've hidden this variable from users and directed them to marketing channels for consistency.
In terms of the set up of this particular variable, Adobe automatically detects when someone navigates to your site from a search engine, it then determines 'Natural' vs 'Paid' via rules that you configure in the report suite manager: Edit Settings > General > Paid Search Detection. (The logic basically come down to whether or not there is a campaign query string parameter indicating 'paid' present on the landing page.)
The "Search Keyword - Paid" and "Search Keyword - Natural" variables are also determined via the same logic. However, these will be largely un-populated as most search engines block adobe from scraping the search term from the referring URL.