Coil Pack

I have a 2000 maxima with 122,000 miles. Recently my check engine light came on and when I had it read I was told that my coil pack had a problem. The car has no signs of a problem in terms of function. The check engine light goes on and off at different times but the message is always for the coil pack. The technician said the entire coil pack had to be replaced at a cost of $800. My questions are "What is the function of the coil pack?" And "Is there any value in replacing it before it fails?"

Thank you in advance for your help