Google Analytics is an integral free tool to help you gauge how successful your website is. Not only that, it can give you valuable insight as to what parts of your website are most successful, and perhaps more importantly, where your website is falling short.
When added to your website, Google Analytics will show you things like how much traffic your website is getting, how much of that traffic is new vs how much of the traffic is returning, and shows the flow of traffic from within your website. This information can be used to see what pages your visitors are going to most so that you can turn those pages into profitable ones.
Analytics will also give you information on what pages cause your visitors to leave the most. This is important to know because the page could be causing you to lose visitors and customers. It is important to look at these pages to figure out what is causing your visitors to leave and what needs to be done in order to fix them.
Google Analytics is truly a fantastic tool. It should be added by everyone who relies on their website to turn a profit.