h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The function of these straps has not been lost. Pulling the shoulder straps closer together and away from the arms has the benefit of allowing the arms more freedom of movement. This is a big pluss when cross country skiing.
If, however, the shoulder-strap-joining strap chafes, compresses your chest and pushes your clothing against your throat you will leave it unstrapped anyways.

Also, with an extremely heavy pack, that little strap does go a long ways to keeping the shoulder straps from coming off. On a day hike, it is as useless as the internal frame itself; but on a 9 day 100 km hike with 30 kg packs it is essential. Plus being able to change the point where the shoulder straps cross the chest from time to time is a great relief.

I've never gone on a 9 day hike with 30 kg packs, but if you want pain and suffering, try carrying 80 litre bags with 8-10 liters of water and the rest air (and a few trout). The water will splash back and forth completely out of sync with your steps and randomly tug on the shoulder straps trying to unbalance you and cause you pain.

or if like me you have shoulders too wide for a womens sack, but too narrow for a standard mans sack (meaning the shoulder straps constantly either try to fall off or dig into your shoulders) the chest strap is an invaluable tool to achieving a comfortable fit....

or if like me, it saves walking around with my thumbs tucked under the shoulder straps all the time.

Good guide entry by the way.