The multichain-cli tool needs to know the right API credentials to use to communicate with the MultiChain API provided by multichaind. If these are not in the default location, you need to provide the datadir. This is because the multichain-cli tool does not find multichaind via some kind of process scanning (remember it can be used to communicate with remote multichaind processes, and it's possible to have more than one node running on the same blockchain on a single server).