You’ve probably heard of the “5 Why’s” method of problem solving. It is a question-asking method of determining the cause/effect relationships underlying a particular problem. If you ask “why” 5 times you will eventually close in on the root cause of the problem.
The “5 Who’s” is a variant that can be used to help with designing your service offerings. Before you offer any service you need to determine who your customers really are. Asking “who” 5 times can help you get to the “root customer” and it is by designing your offering to appeal to the wants/needs/unknown desires of this customer that you can have a more focused offering with greater chance of success.