I'm not sure if I just can't find the primary request for this feature, as I would expect this need to be pretty widespread. Our use case: we have a list for customers, and we need to be able link a new ticket to a customer when the ticket is submitted via a form. As of now, it's a multi-step process which is very convoluted from a user perspective.

It's worth noting that a full implementation of this feature would include the ability to add a new record in the "related" list, rather than just be able to specify a relationship to an existing record.