Whooshka
Fairly faST old guy
I'll throw in with the crowd that says it's not necessary and in fact might be detrimental. I think when people claim it has stiffened the suspension what they are feeling is really increased stiction due to a side load pulling or pushing the fork legs a few thousandths of an inch toward or away from one another. This makes the fork less responsive. The design of the pictured brace does not allow for any accommodation of bike to bike manufacturing differences. The only factory fitted brace I had on a bike was on my VF750 Interceptor and the bolt holes were slightly oval to allow the forks to remain in their natural alignment/distance with no side stress load from the brace. After bouncing the front end, you then tightened the brace bolts last.
Sounds like a good idea to attempt to avoid this problem, but the sequence recommended for tightening the front axle and pinch bolts will result in the distance between the fork legs at the axle remaining the same as before, assuming the front wheel had been installed correctly prior. Probably ineffectual with respect to this particular issue.
Actually it's function is to reduce stiction. The brace between the fork legs actually keeps the legs "linked" at a position other than just the axle and triple clamp, moving up/down in unison, keeping the axle from "bending/twisting" between them. This allows more energy to be transmitted to the rim and tire. Without a brace when you lean into a turn the inside fork leg will assume most of the cornering force and the outside leg will "unload" (I think I remembered that right, might be the other way around ) wanting to be pulled by the axle. The brace makes the fork legs react equally.
I hope I can find the article I read a while back that explains this. It was all about what the front tire, rim, axle and fork legs do.