Google Analytics allows you to analyze the traffic (people who visit your content) that you get on your hubs. You can see everything from where they came from, what keywords they are using to find your hubs. and whether they are using a Desktop or Mobile phone to view your hubs. This helps if you eventually want to monetize your content. You know what your viewers want.
You can do the same thing on Hubpages itself, but Google Analytics shows these things in much more detail.