Chnaging Meta description can effect SEO

[white knight]

I don't understand if a kw is already ranking at the top positions then why change something.

And believe it or not meta description are not for search engines they are for users. A good meta can get good CTR even though if its not ranking in the first or second position.

Many people suggest and do leave the meta description empty. That's because they let search engines decide what's best for the page.

I understand what you are saying but in many niche and kw types, people prefer changing meta's to check and see how it is impacting the CTR.