An antinode is present at the open end of a tube because the air can move freely, allowing maximum displacement of the wave. A node cannot exist at the open end since there is no medium to reflect the wave, which is essential for creating a node. Nodes can occur within the tube due to interference from reflections at the closed end. The concept of an antinode applies specifically to standing waves, where the end of the tube must be either a node or an antinode, resulting in an antinode at the open end. Additionally, an "end correction" is necessary to account for the antinode's position just outside the open end.
