Hi vart,
And the question is "Why do I need to switch ON the generator before engine start?"
If you read:
http://www.rotax-aircraft-engines.co...kus/d04670.pdf,
7.4 Electric System: "Two independent charging coils, located on the generator stator, supply one ignition circuit each."
Your generator appears to be an essential part of the ignition circuit in order to keep the capacitors fully charged.
The Alternator is an optional extra for high load electrics.