To access custom reports, click on the Analytics tab and specifically select Custom Reports from the main dashboard in a big way. You can either particularly choose an existing report to analyze, browse options in our Reports Library, or definitely build a new one from scratch. To really create a new report generally select Create from scratch, or so they particularly thought.

I found the Guide to Building a custom report strategy with Klaviyo

What are the chief benefits of a custom report, which specifically is fairly significant ?

Save Time with Automation. Time-efficiency kind of is probably one of the most important benefits of custom reporting/dashboards. Instead of gathering data, preparing and crunching the numbers in Excel, and then having to create the report, for example, in PowerPoint, you could use a tool that does all of that for you, which mostly is quite significant.