I don't see it as a guard. A guard for what? To keep tiny fingers out? They why only cover only 1/3 of the fan? And besides, PSU cases are not intended to be opened, except by a qualified technician. So the "grill" as seen on the backside of that fan is there to keep exploring fingers out.
Not that kind of guard lol, that's on the outside. this thing is on the inside and it would be like I said, to keep wires out of it (that was the thought behind the comment anyway). Side note: test fingers always creep me out:
In response to the rest of this, I just don't see that being effective with this type of fan. All this will do is create pockets of back-pressure and block off some of the airflow (as well as adding noise).
It won't improve airflow in the non-blocked area. At least, that's what all the testing and simulations I've seen of that scenario would suggest....is what I had typed...which is sort of true, but I think I see the purpose now.
The problem, of course, is to avoid the flow going straight out the back, by-passing the circuit, which was not an issue with the old end placement; I just dislike the idea of a partially blocked fan.
I only got as far as the problem... can't say I have a solution, unless directed slats is an option.
This^ is close and really pointed me to a theory, but it's only half the picture.
I initially had a problem with this statement above as it's not going to just make all the air go straight out the back if the baffle isn't there.
Here's my theory...without the baffle, air is pushed down towards the entire pile of heatsinks and components there, creating turbulence and reflections every which way. Being that the back of the box is vented, the air bouncing around on that side has a relief for the pressure that the fan generates constantly and that air can be easily pushed out the back, but the rest of the air (on the opposite side of the vented panel) will tend to just recirculate (like eddies in a river) and have a harder time finding it's way out the back as the fan is pushing against all of it with constant pressure.
If you create a zone of the supply (right in front of that exhaust-vent) where the fan pressure is removed, it would actually create more of a directionality of the airflow in the tight space as the whole area in front of that vent now also has a directionality of the pressure because the fan isn't fighting it. So while it would be inefficient from the fan's perspective, it does cause more of the air in the PSU to actually cycle and leave (replaced by fresh intake air) instead of just circulating in the corner due to all turbulent pressure from all directions in the box.
I was previously thinking more in terms of ducting and had it in my head they were trying to direct the air from the baffled side to the unbaffled side (like take all the fan's pressure and direct it over here ->), but that's not really it. It's blocking some of the fan's airflow so that the pressure causes the air to move from one side of the box to the other more effectively.
Edit: one more thing to add. As dirtyferret said, 120/140mm fans are quieter than 80mm fans, so this design allows them to get airflow with a more pleasant noise, but they also are just throwing away airflow and fan performance to achieve that result in a way that's more pleasant to the ears. It's still going to be worth it though.
E.g. lack of directing the airflow resulting in need of fan spinning faster.
