Here's my understand of the differences between a main idea, purpose, and central content.
The central content of the passage is basically what the passage is about. The topic.
The main idea of the passage is its conclusion; or in other words, it's author's position on the central content. The point. The main idea is usually stated directly.
The purpose of the passage/author can be any of the following:
- persuade the reader about the main idea
- to inform the reader about the topic of the passage
- to compare something
- to refute a theory
In sum, it's why the author decided to write the passage. The purpose is not stated directly.
I have noticed that the GMAT interchanges these terms.
Your inputs would be appreciated.