The reason for purchasing 100 device licenses is due to the fact that VC in the cluster can change.
VC reports the status of slaves to Central.
So, in case the current VC goes down, one of the slaves will take over.
If this slave IAP is unlicensed on Central, the IAP's for the cluster will be reported as down however they will be working locally.
Service licenses are feature based (eg: presence analytics/guest access/clarity etc).