This looks like a cut-down version of the server rails. The only thing they have in common is the front lock. The cage is meant for ordinary SATA drives.
It means the user has the opportunity to hot-swap a non-system disk: it is quite an ordinary thing to attach a SATA drive on the go. For easier access to the HDD cage there is a smaller door in the large decorative one.
That’s right. Why open the whole panel when you only need to access the HDDs? Otherwise, this system case is an example of a modern mid-format model. It’s got a large air inlet in the side panel:
And it has a traditional design of the rear panel with a 120mm exhaust fan:
The side panels are fastened with thumbscrews, as usual:
They were tightened so hard at the factory that we had to use a screwdriver anyway. If you are frequently accessing your system’s internals, you may want to give the screws up altogether because the panel is also secured with a small slider: