Often users will change their email addresses and sometimes they create a new Atlassian account with the new email, rather than changing the email associated with their old Atlassian account. This causes problems because they can no longer access the Bitbucket content associated with their old login when using their new login, because the content belongs to separate Atlassian accounts. They can't just close their new account and change emails on the old one if they've created content (e.g. repos) in the new one already.
In these situations it would be useful to be able to merge their old account into their new account so they have access to all their content from one account, at which point they can close their old account.