This tutorial is still in draft and is incomplete. Please use with caution.
Tracking users in aggregate monthly cohorts can provide powerful insights. While you can now segment users into cohorts with Google Analytics segementation, you can’t actively take action against that cohort while those users are on your site.
By implementing the cohort tracking through Custom Variables, this allows you to easily behaviorally target users based on their cohort. Additionally, when you use the GA Core Reporting API you’ll be able to extract out the cohort data. That way you’re not relying on the GA interface, which is great but doesn’t always solve all your needs.