But I can't think any purpose :( It could sound silly but yes, it is!.
You'll find the purpose when you find the LESSON that you learn in making the comparison. Only AFTER you write some body paragraphs about similarities and differences can you identify the lesson/wisdom that is hidden in the essay. That is the purpose.
When you write a research paper, it is the same way: Write an introduction that explains the purpose AFTER you have written the body paragraphs.