Floor points are a way of specifying where the floor is when it is not obvious.
The way volume is measured is by determining the space between two surfaces: the stockpiles surface which can be seen and the floor surface which is estimated.
For a free standing stockpile that does not touch anything else, any point beside stockpile is representative of the floor beneath the stockpile, therefore, specifying floor points is somewhat redundant.
However, if a stockpile is touching a wall, a building, or another stockpile, the outer delimitation will not only touch the ground. Interpolating between these points that do not touch the floor can project a completely incorrect floor estimation, which can lead to an incorrect total volume estimation.
As a rule of thumb, if the stockpile is touching something else, it is best to specify the floor points. Here's how to set floor points.