It basically boils down to the typical benefits gained by using a modular app vs a monolithic app.
In 6.2, unused portlets cannot be removed because the code is tightly integrated into the core. They can be disabled, certainly, but not removed to reduce resource requirements. With OSGi, unused modules can be removed from the system altogether.
In 6.2, Liferay must release a big GA to release even minor changes to non-core-related issues. Since it is not modular, all of Liferay must be updated as a result. With modularization, Liferay will be able to release modules independently and not wait for a big GA release.
In 6.2, portlets and plugins are deployed as wars and that leads to big issues crossing class loader boundaries app servers enforce between web apps. With OSGi, modules are deployed within the OSGi container and the class loader issue goes away.
In 6.2 (and earlier), there's a lot of cruft code in Liferay that is necessary to counter issues in all of the supported app servers. In Liferay 7, the app server is out of the picture (it sees Liferay as a single web app only); the Liferay app's OSGi container manages all of the plugins internally.
This is just a short list, but like I said any list of benefits that highlight a modular approach vs a monolithic approach all apply to Liferay 7.
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