Developed before any of today's sophisticated electronics were available, the two-masted yawl could easily be mistaken for a ketch, unless you knew exactly what you were looking at. The ketch, a weatherly sailboat that could stay close to a following wind (if the wind was at 210-degrees at 30 knots, the ketch could stay very close to that course to pickup the advantage of the speed. She would be within 10 or 12 degrees of the main bearing. The ketch also mounted a taller mainmast toward the bow and a second, smaller mizzen, between the rudder and midships).
The yawl, on the other hand, if you were to look closely at it, also ran a mainmast that was forward, but closer to midships. The yawl also mounted a mizzen mast that was astern of the rudder so that one person could easily control her.
To control her, the skipper would set the mizzen and rudder so that they were running with the wind. Then, after setting the mizzen, the skipper could then, but using a rope and pulley system put the take any luff out of the mainsail and fly the mainsail into the wind so that the mainsail, mizzen and rudder, which was controlled by a primitive auto-pilot, remained on the bearing. This is very important in following seas (seas where the waves were bigger than the sailboat and could easily have hammered it into kindling if she'd gone side to and broached) because the yawl could easily show the waves her stern and the seas would then pick her up like a cork and put her down into the next trough between waves again, so she could be picked up by the next roller and carried to the top.
In the 1950s and 1960s, yawls were also made for open-ocean racing. At the time, there was interesting omission in the rulebook and the omission was that there was no penalty for flying a rear-mounted mizzen so the skipper could fly a second staysail. The real advantage in this setup was that the yawl could, due to the mizzen's position at the rudder of the sailboat, stay close to the wind and thus gain a speed advantage.
Where schooners had to fly jibs or spinnakers for extra speed, the yawl could not only fly a spinnaker for speed but could also rely on the aft-mounted mizzen for an extra boost and thus she was effectively flying not only a foresail and jib, but also possibly a spinnaker, but she could also fly a mizzen for even more power.
There's another interesting feature of the yawl and that it is a set it once and forget it kind of sailboat. In other words, once she was trimmed to a bearing and sailing close to the wind on a long reach, all the skipper had to do was tie off the helm and go below. Unless she was hit from the side by a rogue wave, a very unlikely occurrence, unless one was trying to sail through a hurricane or storm, the yawl whose low lines made her look for all the world like a ketch, would remain close to the wind and beat to the bearing chosen.
You could certainly keep a yawl running with a very minimum crews as the famous round-the-world sailers found in the mid-1960s when the Tinkerbell II circumnavigated the globe with just one person aboard.