Thinking of keeping your chin level will translate to a better aligned head and neck, which makes your spot smoother and pirouettes easier.
Your head and neck play a big role in allowing you to to turn and spot. In order for this to be a fluid and efficient movement (that is also easier to balance and continue turning with) the line of your chin needs to be parallel with the floor. When the line of your chin is parallel, it means your head can turn around your neck more smoothly. Try not to dip your chin, or have it too high, or it will throw off your alignment and make it harder to balance and turn your axis.
This tip is really keeping your head and neck better aligned, but many dancers find its easier to think about the position of their chin.
