[OSL | CCIE_Voice] UCM features : why associate user to device with feature if not using EM?

[Juan]

hi all,
 
I'm going through UCM services, but I can't figure out why for certain
features (like barge, privacy, call park...) we have to associate a user
with a device that has these features configured...
I understand the association for user with EM - but the features above are
linked to the physical phone - so using that phone provides the feature
already? Can someone shed some light on this please ?
 
Many thanks for the feedback !
Juan