The primary cause of most imaging artifacts is specular reflection. Specular reflection is highly angle-dependent. Because of this angle dependence, it is important to have an intuitive visualization of incident angles when scanning in ultrasound.
Anyone who has played pool comes to quickly understand specular reflection and angle dependence, employing the fact that the angle of incidence equals the angle of reflection. 
Even though most people feel pretty comfortable around a pool table, introducing geometric terminology and applying this to ultrasound sometimes causes confusion. Reflecting surface and wavefront … wave direction and line normal … these are the foundational concepts from which incident angle is derived.
