What the fences in a diffuser are for
Two separate jobs done by the same vertical plates.
The fences in a diffuser are vertical plates whose main job is stopping the low-pressure air underneath the car escaping sideways out from under the floor before it has finished its work. They do a second job at the same time, keeping the air in each channel of the diffuser apart so that one channel's flow does not disrupt its neighbour, and the two jobs fail in different ways when neglected.
Stopping the sideways leak
The low-pressure region under the middle of a floor sits right next to the ordinary, higher-pressure air just outside the car's edges, and pressure differences push air from high pressure toward low wherever a path exists. The open sides of a floor offer exactly that path, so outside air pushes in under the edges and the underbody flow spreads and mixes with it. Either way the result is the same: the low pressure the floor worked to create is partly filled in from the side, before the underbody air has travelled back through the diffuser and completed the pressure recovery that section provides, and the whole floor behind the leak loses some of its pull. A fence running along the floor's length closes that path and keeps the underbody air travelling backward along its intended route. The leak is worst at the outer edges, where the difference between the low pressure underneath and the ordinary pressure outside is greatest, so that is where fences are concentrated.
Cupping both hands under a running tap shows the same containment problem. With the fingers held loosely apart, water reaching the edges of the palms dribbles off through the gaps, and a good share of what arrives from the tap is lost before it reaches the fingertips. Press the fingers together and far more of the same water travels the whole length of the hands. The fences are the pressed-together fingers. A gap left between two fingers, even a small one, leaks a disproportionate share of the water, just as a short unsealed stretch along an otherwise well-fenced floor edge can undo much of what the rest of the fence achieves.
Splitting the diffuser into channels
Sideways leakage alone does not account for a diffuser being divided into several channels by fences running along its length. That is a separate decision about keeping flows independent. Air moving at a slightly different speed or pressure in one part of a wide diffuser can disturb the flow in the part beside it, and narrower channels with fences between them keep each channel's flow more consistent along its length. Without them, air in the faster, lower-pressure part of a wide channel draws in air from the slower part beside it, and that sideways drift is air no longer slowing down steadily along the intended path, which tends to collect in one corner and detach there first. A narrower channel also gives its fences more influence relative to the air passing through, which is why the channels nearest the centre of a floor, furthest from any outer leak, can often be wider than those closest to the car's edges.
Where the fences go, and why they are treated as performance parts
Fences are positioned and sized according to where leakage and channel interaction are expected to be worst, typically near the outer edges of the floor and around any sharp change in the underbody's shape, instead of being spread evenly across the diffuser's width. They are treated as a performance component in their own right, since a floor with a well-shaped diffuser and inadequate fences can lose a large part of its potential downforce to leakage that never appears in a simple drawing of the diffuser's shape. A team comparing a promising simulation against a disappointing measurement from the real car is often chasing exactly this gap.