It is the ability provided to power user to log work for one of user authorized to log work on a project. Also, it will be possible to correct/complete the logged time from one Team Member.
It allows you to append, as allows JIRA since 4.2, a Log Work Field in Transition Screen. Even if it is a native feature now, you will need to continue with it if you want to benefit from Worklog Type.
It allows you to have, in a Gadget, a short list of Issues for which you can quickly add/correct logged work.