You can use this option to create product groups at your contact.
eg: you have 3 product groups:
3 Online Training
With a contact you can add these 3 where you can indicate per product whether it is Lead, Prospact of Customer. Customer in this example means that he is customer for that product but for another product group is still a prospect.
In your contact menu you can immediately see for which product this contact is already a customer and which product group does not yet.