purpose of waste spark ign

why??

So you can use 1/2 the number of ignition coils. Each coil feeds a pair of cylinders firing at twice the normal rate. This gives spark on one cylinder at the proper time and during the exhaust cycle on its companion cylinder where the extra spark does no harm (or good).

Wasted spark greatly simplifies wiring and you only need one coil driver in the ECU for two cylinders.
The only downside is increased spark plug gap erosion but that isn't as much of problem with modern exotic metal electrode plugs