When a code reviewer checks a big Pull Request, a possibility to compare versions of it after updates can decrease check time and make it easier because a reviewer may not look whole pull request twice.
There is an example: a developer creates a pull request, a code reviewer writes some comments, a developer fixes them and should update a pull request. He fixup fixes with a specific commit and force push it (so a pull request will contain a correct count of commits). As a result, a code review can’t check the difference between the pull request versions from the WEB page.
So I think if Bitbucket will contain a such feature, a developer can use any way to update pull requests and a core reviewer can always have a method to compare versions. Stack overflow offers some ways to do it, but you need to know both commit hashes to compare. A very good example of version comparing - is GitLab. It provides a way to do it (link).
Does Bitbucket contain such a feature as in GitLab?