RENAMES lets you give a name to series of items without needing to have a group item above them. For example,
05 dt-month pic 99.
05 dt-day pic 99.
05 dt-year pic 9999.
01 short-date renames dt-month thru dt-day.
Notice that there is no need to define a group item that includes only dt-month and dt-day, because I can use "thru". For "redefines" you would need a separate group item for those 2 items.
Another advantage of "renames" is that you don't need to specify any pic. It just puts together all the pic's that you are renaming to create a new one for the renamed item. In this example, it automatically comes up with pic 9999 for short-date, without you having to put it in the code.
On the other hand, the disadvantage of "renames" is that you can't change the format at all. That is what "redefines" is for.