It has to do with resort inventory, generally not rooms themselves, and your level of Wyndham timeshare ownership.
Presidential units can be rented by any Wyndham owner who wants to spend the extra timeshare points to book them.
Presidential Reserve units are a special pool of inventory only for Presidential Reserve owners.
https://clubwyndham.wyndhamdestinations.com/us/en/club-benefits/presidential-reserve
I have read some reports that at some resorts the presidential resorts have some extra features, but have not read anything about that at WBC.
I have stayed in Presidential Units a few times, and they were nice, but I wouldn't pay extra to stay in one unless I needed to do a lot of laundry. They have a full-size washer and dryer.