1) Bragging rights.
2) People that have never owned an EV, a large percentage of future M3 buyers, their first criteria they judge an EV by is range. That it is superior in every way except cargo entry opening and headroom will not stop many buyers from putting a plus next to Chevy Bolt and a big minus next to the M3 if the range is less.
3) I don't think buyers will forgo the 300 mile range M3 because there is a 240 mile range Model 3. And if they do and were seriously considering the bigger pack they will likely spend the money on higher margin options anyway.