I have inherited an RPD in which there are the usual physical joins between dimension tables and fact tables, but with a twist that I've not encountered before.

In the Oracle Database tables, the key columns (in both the dimension tables and the fact tables) are defined with a NUMBER(10,0) datatype.  But In the RPD, the data type of those same columns has been manually altered to VARCHAR, meaning that OBIEE treats the joins in the physical layer as if they were between two character columns, not two numeric columns.

Is there some technical specification that requires that the physical layer joins must use text columns instead of numeric columns?  Or, since this is an old 10g repository, was there, at some point long ago, a technical requirement to that effect?  In other words, was the physical layer switch from number to varchar ever really necessary?