Delta's third and final function is to measure the option's velocity.
The delta value represents an estimation of share equivalency. A 0.25 delta option will move at a rate similar to 25 shares of the underlying. In more practical terms, think of delta as a rate of pennies per dollar. For every dollar the underlying moves, the option price should change an amount equal to its delta in pennies.
For example, if XYZ moves up $1, a 0.25 delta should increase by about $0.25. If the price drops by $2, it would be reasonable to expect the price of your call to drop by about $0.50. Keep in mind that this rate is not as linear in reality because it fluctuates as delta changes. As the underlying price rises, call deltas increase and put deltas decrease. As the underlying price falls, call deltas decrease and put deltas rise. There are other factors that can affect delta rates as well.