[Explained]Why Are Google Analytics Dimensions Important? Custom dimensions (which we’ll discuss more later) are different from predefined dimensions in that they aren’t automatically collected by Google Analytics. Primary dimensions are the default dimensions that a Google Analytics report is sorted by. How to Add Custom Dimensions in Google Analytics 4Before using the custom dimensions inside Google Analytics 4, you need to create them first. Because it compiles all your data from Google Analytics, Google Search Console and Semrush in one place.