Once upon a time, the only relationship you could set up in Filemaker was table :: field = table :: field. You couldn't use the other operators such as <, > or X. If you wanted a record in one Table Occurrence to match all the records in another TO, you used this type of field. Since all records have the same key value, all match.
With current versions, we can use the cross product operator "x" to match all records in a table so this is less needed, but still used in many solutions as you have found.
Thank you Phil for the history lesson!
Very interesting indeed.
It seems to me, that it is a very simple way to have access to an unrelated table.