As technology progressed much of the post-production industry moved from film production to video production. Moviolas and flat-beds gave way to a new way of working: the two-machine video editing suite.
In this environment the editor would line up a shot on a source machine and edit across to a record machine. In and out points were marked, tapes were pre-rolled, run up to speed and images in the form of electronic signals were copied from one machine to the other.
The Final Cut Express interface is modelled on the same idea.
Notice the two windows ...