For this example, we're building several packages: core-js, core-css, extras-js, and extras-css.
Using grunt.initConfig, you'll see that our packages are completely intermingled.  In order, you'll see core-css, core-js, extras-js, extras-css, options, core-js, extras-js, options, core-css, core-js, extras-css, and extras-js.
Using grunt.mergeConfig, the resulting config object is identical.  However, each package comprises its own code block, and are completely modular from other packages.  In order, you'll see options, core-js, core-css, extras-js, and extras-css.
Any project that builds more than 1 simple package will greatly benefit from this organizational pattern.
For example, Bootstrap's Gruntfile builds 4 separate packages (bootstrap.js, bootstrap.css, bootstrap-theme.css, docs), jQuery UI builds 3 packages (jquery-ui.js, jquery-ui-i18n.js, jquery-ui.css), and practically every significant project builds multiple packages.