If a Drupal site is running on production with https, is there any reason why this should be done also on local dev like xampp? Just wondered about so many people desperately trying to run SSL on localhost (independently of Drupal).
It depends on the site.
Some third party tech, services, etc require SSL to be enabled and working for basic functionality. Meaning for those cases any/all development/staging environments would need to be running SSL for development and quality assurance.
In all cases the closer your development environments are to the live environment the better quality assurance can be.
If you load something from HTTPS from a non HTTPS site, you might encounter cross domain or same origin errors.
It depends on what you're building and how.
If you need HTTPS locally, Acquia has their dev desktop tool, and I'm pretty sure HTTPS is enabled with a checkmark.
Download - https://www.acquia.com/download