The most obvious effect of spotting is that it prevents dizziness. As anyone who has watched a child spin rapidly, turning this quickly can result in dizziness, making such complex moves as dancing very difficult. By focusing on something stationary as they spin, however, ballet dancers can prevent this severe dizziness. The shoulders turn continuously while the head stays facing forward. When the neck can not turn anymore, the head spins quickly and returns its gaze to the stationary object as quickly as possible.
While spinning, it's important to make sure your body is straight to make sure you are properly balanced. This is another benefit of spotting. Since the dancer is watching a stationary object, she can feel whether her body is straight and firm, unlike if her head was spinning at the same rate as her shoulders. As ballet dancers wear pointe shoes, it is very important that they are confident in their balance.
If a ballet dancer must follow the spin with any sort of movement, she will need to orient themselves in as little time as possible. If she finishes her spin not having spotted, even if she is not dizzy, she may need to take a moment to reestablish where on the stage she is and should be facing. Spotting keeps the dancer oriented as she knows exactly what direction the stationary object is and therefore its relation to whatever direction she needs to head next.
When spotting it is very important to follow proper form. The purpose of spotting is to keep the dancer as elegant during as spin as during other moves, and this purpose is defeated with improper form. The stationary object of choice should be about eye level so that the head is not inclined either up or down. The head should spin at the first hint of tension in the neck to keep the spin going swiftly. The head spin should never affect the body's spin, and should almost appear independent.