Third-party services for Jekyll websites.
Capture email addresses and send periodic newsletters.
Analyse what visitors are doing on your website.
Have non-technical people update Jekyll websites
Content you can drop into your static websites.
Build communities by allowing your visitors to post comments on your website.
Frontend error trackers
Even full backends with authentication are available for static websites.
Add contact forms without any backend coding.
Simple e-commerce is possible on static websites.
Get feedback by adding a survey to your site.
Full search capability for static websites.
Chat directly with visitors.