From what I believe, Google continues through "follow links" to cache the page behind it. "No follow links" are less ideal but can still generate traffic to your website if people actually click on them. Google doesn't follow that type of link however so the only purpose of that link is to generate traffic.
No follow links are not crawled by search engines they are only worked for traffic point of view.Do follow links are the real links crawled by search engines and they are good in terms of ranking point of view.