What is the difference between nLockTime and CLTV? Bitcoin Wiki says:
When the CLTV opcode is called, it will cause the script to fail unless the nLockTime on the transaction is equal to or greater than the time parameter provided to the CLTV opcode. Since a transaction may only be included in a valid block if its nLockTime is in the past, this ensures the CLTV-based timelock has expired before the transaction may be included in a valid block.
According to the above description, it's unclear to me how the parameter to CLTV is used. For example, for a transaction T with nLockTime=400, CLTV=300, the above description basically says since T remains invalid until 400 (due to nLockTime), CLTV=300 must have been satisfied by height 400. True but what's the point? What would be different if T has CTLV=200?