When an operator starts a review of an application, he is automatically "assigned" to this case. No other operator is able to approve or reject the application after it was assigned to somebody else.
Being able to change the operator, or re-assign the case may be useful in different situations. For example, if you have multiple operators or compliance specialists working in shifts, or if a case needs to be escalated to a senior manager.
You can change an operator by opening a detailed view of the candidate / application and clicking on the operator icon. Exact location(s) of the icon is displayed in the screenshot below: