I don't know what version of wordpress is included in the contrib, so you may need to do an update after installing the contrib that you wouldn't need to do if you install the latest version in an ibay.
The contrib installs the application, creates the required database and database user, links them together, and installs some custom template fragments that can be used to easily customize some of the settings in the wordpress configuration file. If you install in an ibay, you will need to manually create the mysql database and database user for wordpress to use, and you *may* need to customize the "ScriptAlias" settings for the ibay (I'm not sure about this).
The contrib installs wordpress in /opt instead of in an ibay. This:
* prevents local network users from accessing your webapp using samba
* prevents the webapp files from having their ownership changed by server-manager (which changes ownership on ibay files if you set a "group" for the ibay).
SME 7.4 uses PHP 4.x; the latest versions of several webapps require PHP 5.x. The contrib will install a version of the webapp that is known to work with SME server.
Contrib developers (some of them anyway, and I would trust dungog on this) will have configured the install to circumvent any known security holes in the webapp. That is, if experts using webapp "X" all know that after installation you have to do A, B and C to make sure your installation is secure, then (hopefully) the contrib has had those steps run so that you don't need to worry about them.