You purchase 10 Nebula Access Points (NAPs) each NAP comes with a 1-year license.
After 6 months you purchase 20 additional Nebula Access Points (NAPs) each NAP comes with a 1-year license.
You now have two different expiration dates on the two sets of Access Points you purchased, as you add more this will begin to get more complex to handle and harder to manage with end-clients.
With Nebula the licenses are automatically adjusted to give you the same co-termination date within each Organization. This helps simplify the management of licensing and the problems often associated with renewable licensing.