Purpose(s) of Field API?

For module developers, what are the purposes of Field API (i.e., why invest attention in it)? (Drupal 7)

From a data modeling point of view, a purpose seems to be to provide a way of creating sub-entities by adding field types to entity types. My initial thought has been that one would get benefits by using the Field API (e.g., security, etc.) as one does when using the Entity API.

However, from the use case examples I’ve seen (example module; book “Drupal 7 Module Development”) another purpose seems to be to allow a drupal user (e.g., admin) to add fields types to entity types via an admin interface.

The second purpose doesn’t interest me so much because I’m at the data modeling stage for my module. My interest is in creating the entities/sub-entities that would help with the logic of my module. In fact, in my particular case, I don’t see value of letting my drupal’s admin users add fields to entity types. This is making me question if the first purpose I described is correct or if I’m mistaken.

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