Your body has time to recover when yu rest. It can't recover to the same extent if your working out every day.
An example of this is with muscles. As you exercise, you make lots of little micro tears within your muscles. When you rest, these repair and in repairing become stronger- they grow. Growing takes calories and energy, you help to create a calorie deficit this way. But if you don't rest, your body cannot recover to the same extent- it recoverd but not as much as it could do so your not creating the same calorie deficit.
However, having said that, there are also other factors to consider. Your not likely to lose 2lbs of fat overnight, but you are likely to lose 2lbs of water over night. Resting requires you to not use your muscles. When you exercise, your blody pumps water through your muscles. It does this for many reasons but onee being to aid the muscles to rid themselves of the lactic acid that exercise creates. You can see this in guys who "pump iron" at the gym and walk away looking "ripped". Its simply water (or a significant amount of it is). This is completely natural but the result ios, your body is holding on to more fluid. When you have a rest day, your body is not pumping water through your muscles and so they are not as plumped up. This water goes through you and your weight drops as a result. Its not fat loss, its not muscle loss, its water loss!