How do we define success?
Success for project A is completely different to success for project B. As we take a problem based approach to building software, the solution and success criteria are defined as part of the process. This is done at the beginning of the software lifecycle but is updated as milestones are reached and business demands change.
How can it help your project?
Perhaps the biggest benefit that comes from measuring product performance is the ability to make data-driven decisions. These learnings can be fed back into the Enhancements (link) that are made to the application. It also allows you to remove the risk of making inaccurate assumptions.
These decisions become critical when iterating on the first version of your product and adding new features. We like to think that every step taken is forwards, but the data helps prove (or disprove) that.