Wikipedia describes EDI as :
“Electronic data interchange (EDI) is a document standard which when implemented acts as common interface between two or more computer applications in terms of understanding the document transmitted. It is commonly used by big companies for e-commerce purposes, such as sending orders to warehouses or tracking their order”
As stated, it is about exchanging information between systems and applications, preferably without human interaction, thus speeding up the information flow, processes, etc. In the end, it is about saving or making more money I would say, but hey, that is perfectly legal.
Having spend many hours getting into the nitty gritty details of all this, we have learned a lot. First of all, it is not one standard, but actually several. Secondly, there are various different document formats. And finally, there is the aspect of companies using specific protocols for the actual data transfer between systems. All in all quite a complex mix.