In the early tetrapod the articulation of the stapes with the skull is different. The arrangement would facilitate the stapes serving as a mechanical sound transducer and amplifier. The finding by itself implicates the early stapes as serving a role in hearing in the earliest tetrapods. Such a role, we can argue, is important in the transition from water to land because sound waves are carried in a medium of two very different densities. Coupling sound transmission from water through the skull to the neurons can be relatively efficient because water has a high density, close to that of bone and soft tissue.
Transmission can pass through bone rather transparently, i.e., without energy loss.
However vibrations in air can only be transmitted through bones relatively inefficiently, much of the energy will be lost because of the large difference in density of air and bone. The stapes, as mechanical amplifier improves matters considerably. Record player needles are mechanical sound transducers and if you have ever seen one, you might have noticed that they are delicately constructed, so that not much mass needs be displaced during signal transduction. The stapes of derived tetrapods, similarly, is delicate. But, as Clack points out, the stapes in Acanthostega is massive, more like the hyomandibula.