The Categories module is a very power feature in EE, but the publish entry controls for category selection fall a bit short. Specifically, the way it groups all category groups together in one field type. This becomes more of an issue when using multiple category groups per channel and placing them in logical order with other custom fields is a requirement.
This field effectively solves this problem by turning category groups into a custom field with custom instructional text and placement anywhere in a publish form.
As a bonus, long lists get a filter option to help your clients quickly find categories in a long list. If enabled, the edit link is also placed below the list for quick access to the category management tools.