Exactly. My 1911 is 8 in the mag, 1 in the chamber, cocked, locked. Why anyone would not have one in the chamber and a completely full mag is beyond me.
Sometimes one may wish to transport or store an unloaded firearm along with a loaded magazine. If the number of rounds one will want in the gun once it's loaded is equal to the number that fit in a magazine, the firearm and the fully-loaded magazine would represent two items one would have to handle. Adding another round would be a third item.
A more interesting question is why people carried six shooters with five rounds in them, rather than having a "parking notch" so the hammer could sit between chambers until the firearm needed to be readied for use. Until the gun is cocked for the first round, the hammer would sit between chambers rather than on a primer. Once the gun was fired, the hammer would be sitting on a spent primer. I know North American Arms uses such notches on their mini-revolvers, but it would have seemed an obvious improvement 100 years before.