if a user has two device and if u assign same owner-id for both phone CUCM will count it as enhanced plus license.
if three devices are assigned with same owner-ID the CUCM will count CUWL license.
if u assign Owner-ID and the user is using only one device CUCM will count the license based on the phone model. and even if you keep anonymous CUCM will count the license based on the phone model in this case.
Owner-ID is used for CUCM to properly allocate the user license.