The idea is to be able to have all events in a visit tied to an eVar even if those events occurred before the eVar was set. This can end up being extremely powerful.
Client wants to distinguish video views between customers and prospective customers. Their only way to do this is by using an eVar to describe the visit as a logged-in visit or not. Unfortunately, if a user first views a video, then logs in, then views a video, only one video view is attributed to coming from a logged-in visit. If the eVar was set to capture all events in the visit, then both video views would be allocated to the eVar value of logged-in.