Motopacing mimics the power and physiologic demands of racing in a pack.
If you are familiar with riding in a pack, then you know how much surging goes on; a constant mix of hard accelerations, coasting and some steady pacing. However, despite all of these peaks and valleys in power output, one can have a relatively consistent heart rate/aerobic response (due to the interplay of brief work and recovery) when the pack is going hard for extended periods of time; and indeed, this is exactly what happens when you work to maintain your position within the draft of a fast moving motor vehicle.
When riding solo, power output will oscillate around the average by 10-20watts while the road gradient is consistent, with a bit more variance for brief moments when adapting to gradient changes. When riding in a pack or motor pacing, power output will vary to a much larger degree; as much as several hundred watts above the average and all the way down to zero. It's the average power over time that will cause the physiologic response to be the same, while the nature of how the power is produced will have differing effects on one's ability to quickly change speeds (like riding in a pack) or holding a steady pace (like climbing or time trialing).
In the end, an advantage to motopacing is simply that it reproduces the unpredictable and variable power demands of racing in a pack under conditions that you can control, including intensity and duration.