Forge ring allows you to have two slotted items of the same type on your person (i.e. you can have two rings functioning happily alongside each other), where in all other cases you can have only a single item in any given slot.
On a similar note craft rod allows you to create non-slotted items (rods) that have market value of slotted items. (That is, rods don't use one of the body slots, but do not have the ×2 price modifier similar wondrous items would have.)
OTOH, rods cannot be always active, and rings can. Perhaps these are the reasons why the drsigners thought rings and rods needed the separate feats to create them? I wonder...
Regards!