Ledges like these could be covering pipes and conduits. Sometimes it's not possible to integrate these into the floors or walls (or they've been applied after the house was built), and this is a convenient way to both remove them from sight and provide a basic shoe rack.
If you are very curious, you could try removing the top cover. These tend to be placed without any kind of fixation (and judging from your photo, the cover is mechanically supported on at least the front and sides, very likely also the back), so you should be able to lift it immediately. If not, gently pulling the front ledge upwards (you can start on one side) may release any nails used to secure it.
You could afterwards choose to remove any nails and just place it back without fixation (which is not really needed), or alternatively just drive the existing nails back in with a hammer if you want to prevent small children and pets from hurting themselves on anything you find inside.