Being able to apply a base shift requires that you are able to use a rover on the Known Point in order to calculate the shift.
Take this example. You have a Known point on the sidewalk at a busy intersection near where you plan to survey. Instead of setting up your base on that Known Point next to traffic, you want to set up your base in a parking lot nearby instead, but you want all the points that you collect to reference that Known Point, not where you are setting up your base in the parking lot. You could use the base shift workflow for that.
Read through the guide and it should make sense.