I don't know anything about that aircraft, but an alternator requires in initial charge before it can start working. Normally that will come from the battery. However it does mean that if it is turned on, and the engine isn't turned on, it is eating up some of the battery's output. Therefore to get maximum power from the battery at startup, an alternator is traditionally left until the engine is started.
A generator doesn't need an initial charge so doesn't have that limitation.
However a generator isn't as efficient and is likely to require more maintenance.
No idea why that aircraft has both.
See a short description here:
The differences between a Alternator and a Generator | Ask a Flight Instructor