In velodromes the track shape has an influence on the drag measurement due to the lean angle of the bike.  Knowing where the bike is on the track allows us to account for this and give a highly repeatable measure of drag from one lap to the next.
Out on the road, one of the biggest sources of error is the altitude measurement, which we calculate through measurement of atmospheric pressure.  Although Aerosensor has industry leading accuracy in this respect, it is still not perfect.  Using two lap triggers we can get a much better altitude measurement between your start and end points, improving the CdA measurement.
aerodrome sends the lap start/end signal direct to Aerosensor with an accuracy better than 1ms, allowing the internal drag calculation to be synchronised with track position to a very high degree of accuracy.