An adaptor solves a purely mechanical problem: a still's outlet, whether a spout, a bent neck, or the end of a worm, rarely lines up neatly with the mouth of whatever receiver is meant to catch its output, especially once an operator wanted to change the receiver's size or shape between runs. The adaptor is simply a short tube, bent or curved to whatever angle the setup required, fitted onto the still's outlet at one end and directed into the receiver's mouth at the other, carrying the condensed distillate across the gap between the two vessels without any of it being lost to dripping outside the receiver.
Because it added no heat, no cooling, and no chemical step of its own, an adaptor's only job was geometry: letting the still and the receiver sit at whatever height, distance, and angle suited the bench and the size of the vessels being used, rather than forcing every setup into one fixed arrangement. Some adaptors were built with a second branch, letting an operator swing the flow between two receivers without disturbing the still itself, useful for collecting different fractions of a distillation as they came over without having to stop and swap a single receiver mid-run. A well-fitted adaptor also made cleaning and reassembling a still far less disruptive, since the receiver could be lifted away and replaced without touching the still head or the vessel underneath it at all.