Why use multicall.js instead of web3.js?

[wjmelements]

There's no advantage of using your smart contract vs regular jsonrpc batching. In fact the performance is worse.

Get the return value(s) of multiple smart contract function calls in a single call

This is not superior to using a batch because the batch is in parallel.

Guarantee that all values are from the same block

You can specify a block number in the call. Polling "latest" results in unnecessary calls since you should only poll when you get a new block. A block is already a snapshot.

Use watchers to poll for multiple blockchain state variables/functions
Get updates when a watcher detects state has changed
Results from out of sync nodes are automatically ignored
Get new block updates

These are valid and useful, but don't require the batch contract.