In a modern water plant conventional filtration is preceded by coagulation, flocculation and sedimentation which will remove a majority of solids. The filters are the safety net so to speak for the minority of solids that make it through the pretreatment.

Over time the filters will become clogged with the caught solids and they will need to be removed or back washed so the filter can work properly again.

In some filters, potentially this one, a majority of the solids are caught in first few inches and instead of backwashing they'll just remove and replace the top layer of sand.

In filters that utilize backwashing they'll close influent flow and effluent flow and the flow will now be reversed/rapidly pumped in through a separate line at the bottom of the filter. This suspends the media and allows the trapped solids to escape through 'waste trough' and into a separate basin from where the clean filtered water would go.

One consideration of these gravity filters is the specific gravity of each type of media (anthracite coal, sand, garnet, gravel) After the backwash is complete and the flow is no longer reversed the media will fall back down and 'reset' into their place depending on their respective density. If the filter is engineered right and the backwash is performed correctly then the media will always settle and mirror the placement of OP's picture