Custom dimensions and metrics let you collect and analyze data that Google Analytics doesn’t automatically collect.
Custom dimensions and metrics can easily help track and analyze data that matters to your website.
Google Analytics tracks specific attributes and metrics by default. Some examples of the predefined dimensions are age, region, product, etc. And some predefined metrics are bounce rate, page views, and average time on a page.
With custom dimensions and metrics, you can track more data that you have on your site. So, for example, if you run a blog, you can add “Post type” to see which posts have the most views. Similarly, if you run an e-commerce site, you want to add elements such as “Logged in” to see the interaction between the logged-in user and your website.