Whats the difference between primary , penultimate and main purpose of an essay?

in simple words
main idea and central idea is a same thing which means what idea you get from the passage i.e. the primary purpose.
PENULTIMATE is what the SECOND LAST paragraph of the passage saying.

Thank you , it is penultimate paragraph,I typed it wrong. What does the examiner mean when they ask about primary purpose of any paragraph or an essay?

Hi Bipul,
Could you please cite the source where you came across these terms? The questions most often involve the primary purpose and/or the main point; however, we have never come across the term 'penultimate purpose.' We often use the word 'penultimate' to refer to the second last paragraph.

So central and main idea of the passage is same right!?

Primary purpose means why the author wrote the passage, what is the central idea of the passage.