Some custom data use cases require storage a hyper link as a custom field value. For example the organization entity has field to store its web-site URL and display it as a hyper link. But it's impossible to display such URL as a hyper link using a custon data field.
I need to store a hyper link to data stored in other services. It can improve integrability of civicrm vary well.
It can be vary usefull to store one custom group within other custom group. It can simplify of reading and understanding information stored in custom groups.