Under normal conditions, there is no need for you to call start mining manually at all as the node calls this method instead of you during the start-up process. However normal conditions are not always present and these few edge cases when you must call start mining manually are described here
If node start-up process fails
As we all know sometimes unexpected things happens in software, you might successfully boot-up your node, but other nodes from some reason did not receive AccountMining event emitted by startMining function that you just automatically called. As a result, you were not voted as validator and therefore you cannot take part in mining process and earn rewards...
If this should happen, you will simply notice that even though you vested in chain and joined network as a validator, you are simply not receiving any rewards + after some time you will see on the terminal message: As of block: ... you are not registered as validator on geth level. Please control this by attaching to the rpc port of your node and call istanbul.GetValidators(). In case your node is running for more than 10 minutes and you are not voted as validator, please call StartMining on Lition SicechainManger here: https://www.lition.io/sidechainmanager/ and you will be automatically voted as validator by other nodes.
You can validate that it is really what happened by Reloading Users details in the left panel and see if Mining parameter is set to true