This is a stupid question. First of all even if these were all true, their not reasons it's a psd file, they're just descriptions of what a PSD file is. There are plenty of applications which can open and write psd files.
There are plenty of other file formats which have layers, it's not unique to psd
What the hell does "it can be saved as other file formats" supposed to even mean?
So basically this question has brought up all the things that make a PSD file the same thing as any other file and hasn't brought up anything which makes it unique.