For example: running `brew install php@7.4; brew upgrade php@7.4` the first (left) command can fail in case the current version is unavailable or due to the local brew dependencies problem. In that case, the right command will be invoked anyway and of course, will fail too.
If we chain the commands with the `&&` instead, the execution will stop at the first failed command and we will not execute the next one in the commands chain.