Preview is preview, there’s the preview view labeled “Preview” which shows the preview of the model(s). The tickboxes show/hide each 3D model in the list.
If one model is completely smaller than the other it won’t be shown because it’s inside the larger.
The model can be shown only if there is an actual file in the filesystem. See Why a footprint points to a 3D model but the model file doesn't exist? and Add an indicator to the Footprint Properties dialog that the 3D model file doesn't exist (or there's some other problem) (#3815) · Issues · KiCad / KiCad Source Code / kicad · GitLab.
It does make sense when you want to compare two models. Otherwise you should always delete and add paths again when doing comparison.