The CAC/PIV card is a more durable card with an embedded chip that stores biometric/electronic data. this would be useful for the following purposes.
electronic signature
benefits eligibility (government / medical / insurance)
electronic voting
driver license (any type of license / permit)
concealed carry permit
WIC benefits
food stamps
building access
TSA data
It would create a 50 state standard identification that would make numerous things easier and reduce the number of government issued IDs and the systems for maintaining them. Military and Federal workers already have them and they are infinitely useful. Even on this website I can see where I could sign a petition 5 or more times if i register every member of my family and sign for them. If I were to use a CAC/PIV I could not.