truffle-assertions | Assertions and utilities for testing Ethereum smart | Cryptocurrency library

Solidity introduced automatic exception on integer overflow/underflow in version 0.8.0.

Arithmetic operations revert on underflow and overflow. You can use unchecked { ... } to use the previous wrapping behaviour.

Source: docs

If you're using Solidity ^0.8, it's still going to throw on balanceOf[msg.sender] -= _value; (assuming value is larger than balanceOf[msg.sender]), because otherwise the balanceOf[msg.sender] value would underflow.

You'll see this exact check using the require statement in many sources because they were either created before the version 0.8, or their authors might have not been aware of this new feature. Or they maybe wanted to pass a custom error message using the second argument of require(false, 'error message') (the auto exception doesn't give any message). But since 0.8, it's not needed anymore.