Well I think its because most of BMW cars reuse names, for example 3-series could mean any car from a 1980s 3 series to todays car. Also the actual model is useless cause there were 318is back in the 80s as well as right now. So the best way to identify these cars is by their chasis code. In 20 yaers from now there may still be a Z4 driving around, so it would be nice to know which version we drive
