It's an attachment which is a single point and a rotation. This can be used to spawn projectiles, particles, etc. A bone can be used in similar ways, but a PointAttachment is slightly less expensive to compute and can be hidden, shown, and placed in a skin.
Imagine you have multiple guns in a slot which spawn their projectile from a different position. You could have another slot with point attachments, one for each gun. When you attach the gun, you'd also attach the appropriate point attachment.
They are useful for many other things. Also, future functionality may be very similar, such as a BoundingCircleAttachment which is a point in space plus a radius.
