August Horch, who managed the production of the motorised vehicles for a longer period at the Benz company, describes the ignition system in his book 'Ich baute Autos' (I built cars). It was a battery ignition, which, even at that time, worked with high-voltage. Being in the cylinder head, it was very difficult to insulate and moreover was exposed to very high temperatures, one already chose porcelain from the Royal manufacture in Berlin. As you can see, certain details in the motor car have asserted themselves, right up to today.
Bosch's high-voltage ignition had of course, an enormous advantage over that used by Benz, it didn't need a battery. At that time, the generator had not even been invented, this of course, limited the working range of the vehicles. It's the same as with todays electric cars, after each trip the battery has to be re-charged. Only after the event of electric light and (in Europe, only occasional) starters in motor vehicles, would the generator actually be necessary, thus making the battery ignition meaningful.
It was presented in 1925, after Bosch had already been famous for a long time as a specialist for reliable and race-tested ignition systems. It promised more reasonable production costs, thus playing a part in the, albeit much later in Europe, mass-production of the motor car. In view of all that, it's not surprising that they were first installed in series by GM in the USA. It's triumphal march only started In Europe in the period between the two world wars.