When is the filter value available to a context filter in a view?
At the very top of the contextual filter configuration window you have:


  This display does not have a source for contextual filters, so no
  contextual filter value will be available unless you select 'Provide
  default'.


The answer to your question When the filter is available? is short:

The filter is available only if has been provided.

Let say, you want to show a block with the content that depend on node id. Obviously you don't want to provide the node id by yourself on each node to see what the block content is. You don't need to provide it, because the node id is in the URL, so it's available. Of course the node id is not always in the URL, for example if you're viewing a user profile or a list of recently added articles. That's why this fieldset has been labeled:

WHEN THE FILTER VALUE IS NOT AVAILABLE

In this fieldset you can decide what to do when the filter hasn't been provided (display all results? provide default? .. many options there..)