Why German designers considered mixed diesel and steam propulsion

Why did German designers consider combining diesel and steam machinery?

Diesels could improve range, but steam was still needed to reach the desired high speed.

The engineering attraction was endurance rather than a complete improvement in every performance measure. Contemporary diesel proposals struggled to deliver the required maximum speed by themselves. Steam machinery could supply the missing high-power performance. A mixed plant therefore appeared in the design debate as a compromise between economical range and tactical speed.