Spaces having plan views but not section views?

For the same reason that anything might not show in a section and will in a plan view or vice versa. You can't show height in a plan view and you can't show width in a section view, so you need both. For example, a furniture partition might show up in a section view as blocking a view, but look like a low wall in a plan view. The reviewer needs to see it in section to know the height. Am I understanding the question correctly?

It helps verify your sill heights and ensures that any permanently mounted partitions or casework are not impeding the views at either sitting or standing levels. Your cut height for floor plans is above the 42", so if your sill is 45", then it would still appear as glazing in a floor plan view. However in section it would not be valid. Same issue with counters, typically they are not shown as cut in floor plans, but the height can be greater than 42" therefor impeding the views of occupants seating.

You can look at the requirement this way, its a check for the design team to ensure that the occupants truly do have a view. You don;t just design in plan view since we live in a 3D world.