If a module or library has been added to an application, it was done to serve a purpose.
It must add some required functionality that was asked for earlier in the application's lifecycle.
It was needed then.
But it is still needed now?
Are the original requirements still valid?
Have they changed or are no longer needed?
If the code is no longer serving a purpose, it should be removed.
Common culprits in Drupal projects are the Feeds and Migrate modules, which are used to import data from external sources, such as legacy applications when migrating to Drupal.
Once the data has been imported and the site is live, the modules are often so longer needed.
As I said yesterday, having less code in your application will make it easier to maintain and upgrade in the future, so keep it as lean and minimal as possible - but this is an ongoing process as requirements change over time.
