What change does the OS need to make, given this message? "iCloud Preferences wants to make changes to your account" This message appears when I log on to my machine. It doesn't tell me what change it wants to make, so I deny it access to my system.

Does anyone know what change it is trying to make?
It's trying to set up the iCloud account as a trusted authentication platform for your local user account. This is usually for a feature such as the Apple Watch unlock, whereby the Apple Watch can unlock your local account through iCloud.