Sometimes in the middle of a build you realize you placed a piece wrong and you need to remove it.
In the case of a plate that is surrounded by other plates this gets a little tricky. Say we need to remove this green 1×2 plate.
If the plates are snug against the build plate, there is nothing you can do but remove the other plates around it to get to the one you need to reach.
Even with that gap in the surface that a separator will fit in to, the plate behind it will not allow the separator down far enough to remove the plate in question.
If for some reason you have not pushed the plate the whole way down and there is a gap, you may be able to wiggle the other end of the separator into the gap to remove the plate.
In most instances this is not the case. Removing the plates around it is the only way to access the one you need to remove.
Should the plate you need to remove be in the middle of a much larger build you would need to remove enough plates so that the brick separator would be able to go flush against the bottom plate and the width of the plate you would be removing.
The brick separator is basically 10×2 studs but if there are other plates or bricks against what you are trying to remove they will stop the plate from tilting enough to be removed.