They are ground posts.
It is an Intel D845PEBT2
According to the spec sheet for that board, it follows the ATX 2.1 standard for mounting holes. The ATX 2.1 standard for mounting holes quite clearly states that the mounting holes are for "mechanical support only". You'll note specifically that the "only" means that it doesn't do anything else, such as being electrical/ground support, which would be utterly stupid and dangerous.
Also, in every board I've seen that features the metal "bracing" around the screwhole for mounting, the metal on the board is Aluminum or brass, or some other non-conductive alloy.
Geek: if you look closely at your power plug, you may notice that it has a third prong. Modern devices use this as their electrical ground. And if you removed that prong because you are living in a old house and somehow think your case and motherboard magically retrofits itself to use aluminum and brass to conduct ground voltage, well, I'm not really sure where to start.
If computers used the motherboard mounting holes and case for grounding, that would have two issues:
1st, the entire bloody thing would be a electrical and fire hazard. And what if the case is plastic? There is no provision in ATX that the case be metal, let alone a metal that conducts electricity; otherwise there would surely be documentation on the minimum conductivity of the case. How would a board conduct ground through a plastic or aluminum case, for example? particularly using a aluminum or brass standoff, as they commonly are, to mitigate the possibility of the very thing you state happens by design.
2nd: where does the power supply get it's ground from? It's own case? But what if the case it is installed in is not a conductor? Then what? I submit the clearly preposterous idea that the power supply get's it's ground from this magical thing called the "GROUND" prong on any modern electrical socket. If you look, you'll notice, quite clearly, that all power supplies require the presence of this prong. Additionally, this grounding is passed along to the motherboard along the clearly confusingly named GROUND wires.
If I recall, however, you yourself have documented that you "hacked" your power cables because you didn't have a three-pronged cord to use. This doesn't magically mean that your computer will use the case as ground, even if it is a conductive material and your standoffs weren't made of brass or aluminum which they always are. It just means that none of your voltages will be measured properly, and who knows what else. I certainly wouldn't trust a machine in such a position.
Electrical power running to the case is not "being grounded" it's called a "short" and is not desirable, and can result in injury or death if mishandled. Or in the hands of somebody who thinks brass standoffs can conduct electricity through aluminum, and then through silicon, as well as a computer magically being earthed.
