Hooks fire, allowing other modules to hook into the process at various points. These modules can change the content returned, add business logic and extra functionality.
An example of this is adding comments to the blog post. The Node module provides a hook in point for other modules. The Comment module utilises this and attaches comments to nodes. This is where the true power of Drupal is, because modules can provide any number of hooks for other modules to implement.
There could be hundreds of hooks implemented in modules, all able to alter the content.