Instead of receiving sometimes garbled data from a group of aircraft it enables controllers to deal with aircraft on a one-on-one basis, giving them clearer and more precise information to work with.
I might be wrong, but I'm not sure that's quite right. The "garbling" issue isn't about the controller but the radar system. My understanding is that basically prior to mode S the transponders on all the aircraft in the sky are all sending data back to the radar receiver at the same time, sometimes if there are too many aircraft all transmitting at once the radar receiver can't cope and isn't able to decode all the information it is being sent and present it to the controller in an intelligeble way on the radar screen. It's a bit like if you're in a room and several people all try and speak to you at once - if you were to try and work out what they were all saying simultaneously you'd most likely fail to actually work out what any of them are saying, your brain can only cope with one (or at best two if you are an amazing multitasker) person speaking to you at a time. In short, trying to concentrate on too many pieces of information at once results in you taking none on board, it all becomes "garbled".
Obviously when a radar system experiences that problem its not very handy for the controller who needs that information to do his/her job. Going back to the analogy, the best way to communicate effictively with all the people in your room is to listen to them one at a time. I believe this is what Mode S is doing. It avoids garbling by focusing on the return of one a/c, getting it all down without any confusion, then moves on to the next one, and so on and so on... I believe Mode S does this by actively interrogating individual transponders, but am prepared to be corrected by someone who knows a little more.