In following code:
console.log("Deploying contract....");
const simpleStorage = await simpleStorageFactory.deploy();
await simpleStorage.deployed();
Line 2 deploys the contract and we get hold of it.
Why do we need to call deployed method in Line3?
calling deploy() will create the transaction, and you can get the contract address immediately, however this does not mean the transaction has been processed and included within a block.
deployed() will wait until it has been. Under the hood it will poll the blockchain until the contract has been succesfully processed. See: https://github.com/ethers-io/ethers.js/blob/master/packages/contracts/src.ts/index.ts#L824
I don't think you technically have to call deployed(), but if you need to do anything after the contract has been deployed and need to make sure it has been included in a mined block, then waiting for deployed() is advised.