Sometimes it is useful, if views are "coupled" with respect to
their "point of view". The classical exaple is the CAD-like
axial/sagittal/coronar 2d view sets.
These views are couples so that you always see 3 views that
intersect at a given point.
This point is the "camera".
As you can see, the camera only specifies, where you look _from_,
but not, where you look at.
Another example is the "rearview mirror" setup that e.g. colotux
uses. Whenever you move one view, the corresponding "rearview"
will also be updated.