When the camera is set to HDR, if not post process are running then the camera fallback to LDR without any warning or messages.
As post-effect are not delivered by default, this is bad because when first opening up unity even if hdr and linear are set some strong banding will occur because of LDR -> not cool for 1st impression.

-> summary HDR button should do what it says (OR we should print a warning at least.)

Repro :
open attach project
load scene testScene -> notice the banding / the LDR rendering (using a frame debugger like renderdoc)
activate dof on the camera -> banding is less strong (16b buffer are used)
activate tonemapped -> banding is gone (16b buffer + dithering is applyed)