In a relational database, foreign keys are normally used to associate records stored in different tables, but wouldn’t it be nice to define relationships dynamically without having to add extra columns or tables? And while we’re at it, how about having sparse relationships by associating a record directly with any other record like “post X was last edited by user #123” or “post X was flagged for review by user #456” (who happens to be a moderator)?
There’s certainly the danger of creating too many ad-hoc or sparse relationships, which can cause issues. That said, when used for supplementing foreign keys, Tie-in can be a useful tool in a production system as well.