You are right. It’s intended to harden the network against a few forms of attack and to protect a minter’s block reward.
The post block solving stake lockout period (5020 blocks, ~3.5 days) is intended to make sure that the shares granted as the reward for finding the block are part of a transaction that is buried “deep” enough into the primary network chain that it won’t be displaced if the current primary chain is replaced by a different, longer one.
Peercoin uses an almost identical model, with one difference. Due to the fact that Nu uses a one minute block spacing vs. Peercoin’s ten minute spacing, the number of blocks required to be confirmed before a stake reward is released back to the address is 5020 versus 520 as in Peercoin.