Well, I'm just too late to this, then.
As Brucey says, this is one of the most highly stressed bits of your frame. A fork which is designed to take a load in low-rider panniers and/or a disc brake can put a fair bit of force through here.
If you are pushing the design/specification in terms of loads applied, then you can demonstrate that this gusset adds stiffness. That doesn't mean you need this gusset for a gentle pootle around the lanes, but it might mean that you can ride home after an "off" instead of getting a taxi, or that your load-lugging bike will last as long as your "Sunday best" before the frame fails.
Frame failures on Spa bikes are uncommon, and I can't call to mind any failures at this joint (crosses fingers!).
Our biggest generator of failures was one builder whose braze-ons had a habit of coming off, and their welds occasionally started fatigue cracks; they don't build for us any more. Somebody thinks broken frames won't get billed back to the builder in the Far East, I can assure you they do! It certainly isn't in the builder's interest to have frames fail.
Theres not a lot of choice in 725 tubes, so I don't think that's a factor in different manufacturers choosing to use a gusset or not, see https://www.torchandfile.com/assets/images/PARTS%20LIST%202019.pdf
Why is the bottom of the gusset un-welded? because the last thing you want is a line of weld across the tube at that point. Have a look at a few lugged frames which have had a front shunt, the downtube is creased just at the end of the lug. Lugs are thinned and pointed to eliminate stress raisers, and gussets are open-ended and scalloped for the same reason.
My bottom line is that a gusset adds strength and stiffness for negligible weight and cost. Whats not to like?