We really need to add a documentation page for point attachments. It's a point in space and a rotation, which is transformed by parent bones like you'd expect. This can be used to spawn particles or do anything else that involves a position and/or rotation.
You could also use a bone for this, but there is one main advantage to using an attachment: a point attachment can go in a skin, so the position and rotation can change for different skins. For example, different guns may shoot from different positions, so you could use a point attachment for where the projectiles spawn.