Indeed, it's to stop the two separate pieces of the rack from twisting around the fork (keeping them parallel to the wheel) Notice that these style racks only attach to the outside of the fork. Most front racks that don't have this bar attach to the inside and outside of the fork at the eyelet, but some bikes only have a threaded eyelet on the outside, in which case these style racks (with the bar over top) are a better option.
notice here is a style without that bar and attaches to both sides of the eyelet on the fork:
http://www.biketiresdirect.com/produ...der-front-rack
vs this one that has the bar overtop and only attaches at one point on the fork:
http://www.biketiresdirect.com/produ...der-front-rack