The Jumo 004 was the first jet engine used in combat (powering the German Me-262 jet fighter, as well as a variety of prototypes of other craft), and the first to be produced with an axial-flow compressor. Thanks to the axial compressor, the engine’s got a much slimmer profile than do jet engines with centrifugal compressors (for instance, the J-33 engine). As a result, the Jumo could be slung under the fighter’s wings (vs. being buried inside the craft’s fuselage), greatly simplifying maintenance and replacement. Given that engines of their time could run for no more than about 25 hours between overhauls, this was a very helpful thing.
The axial compressor, though, came at great cost — it took so long to control turbine blade vibration issues that the Jumo 0004 was only available in any quantity in the early months of 1944, nearly 7 years after the engine design started. This played no small part in limiting the impact of the Me-262 in WWII.
Still, the Jumo was a big step forward in jet engine design, and helped lead to many of the features seen in modern jet engines.