A reviewer can look at a pull request, and if they see room for improvement, instead of leaving a passive comment they can suggest a code change that can be then accepted by PR author with one click.

Original comment:
Github added this feature: https://blog.github.com/changelog/2018-10-16-suggested-changes/

And I would really love to see this also in Bitbucket, would be very very good for productivity of the developers.

I found this similar ticket: https://bitbucket.org/site/master/issues/17360/allow-users-to-apply-suggestions-to-pull