Historical ecommerce data are all data points since your ecommerce site went live. It includes all sales, marketing, customer and other data. It’s important to use an ecommerce analytics tool that makes this all-time data available and usable because, first, it’s yours, and second, it is insanely valuable for your marketing.
For example, visitor sessions from day 1 can be very useful when the visitor registers with an email and you can reach them now, using the behavioral insights you get from their browsing your site - what categories they’re interested in, where they’re located, etc.