I had to torque some 3/4" driveshaft bolts. When they were simply tightened by wrenching (or double wrenching) the bolts would be torqued inconsistently, causing caps to bind and throw driveshaft. It was actually more of a u jointed "axle shaft" and would gouge and rip up the "cross drive" case aka transaxle housing.
You use them when you need to torque a bolt that a socket/ratchet won't fit on. You might also use them as a crowsfoot for removal of a bolt where a socket doesn't fit but a wrench isn't long enough or needs some kind of flex like a breaker bar.