Locaction. They're close to El Centro and the resort is huge.
Alternatively, you can request to be in Cabanas and pay the standard room rate and be about as close to El Centro. However, the Cabanas are a very popular request and are, therefore, often not granted.
So one might think of booking a preferred room as, more or less, an insurance policy of not being put too far away from El Centro (e.g., in the Ranchos).
Another advantage could potentially be with the buses because you don't have a terrible walk to get to either stops 1 or 4. For example, you could get off at Stop 4 (first stop coming in to the resort) and get on at stop 1 (last stop leaving the resort). This effectively makes CSR a 1 stop resort (as opposed to a 4 stop resort) as far as you're concerned.
Getting on at stop 1 works particularly well if you're an early bird (uncrowded buses), especially if you want to stop in for breakfast/snack in El Centro before heading out to a park.
Another thing to note is that a high percentage of preferred rooms tend to be booked by people on business due to their proximity to the convention area (for some, this detracts from the Disney magic).