It's related so there must be a link somewhere in case the record get updated. Depending on the type of data, the link would not necessarily be more memory expensive compared to the value: such as an image, a big html content. My wife used to write her meeting schedule on a postit and put it on the fridge as reminder and she doesn't want to listening to my lecturing on single version of truth only to find out sometimes the meetings are rescheduled and she misses them. In ERP systems like Odoo, data consitency is of crucial values compared to be economical saving a few kb here and there.