Add Relationships field type in custom member fields. One use case would be is when a site has a list of products which is used across the site to relate entries, this could be used to relate members with products owned.
A user that registers could select their product (Custom Member Select Field related to Products) form a list to show which product(s) they have purchased.
Not sure what the need is for the masses, but I could use that and can see how that would benefit a lot of folks!