As Pilot DAR says. It's a lot easier to start an engine with the gearbox decoupled and then progressively couple it once the engine speeds up to idle. Similar thing happens in car with manual transmission - you never start the car in a gear with the clutch coupled. Having the clutch has another advantage - in case of prop strike, it decouples the gearbox from the engine and thus you only have to do a shock load inspection on the propeller and gearbox - saves money. On the other hand, if the infamous Thielert clutch fails, you end up with the engine delivering power/torque, yet none of it (or not all of it if the clutch has just started slipping) is delivered to the propeller.
Originally Posted by Jan Olieslagers
For a beginning: I think it could be said more properly that the DA42's engines have a clutch. The DA42 being a twin, it actually has two clutches.
Let's not confuse anybody - each engine in a diesel DA42 has a single clutch. There are engines used in cars which have two-clutch transmission (for a single engine), but this design would be completely unnecessary for a single-gear reduction gearbox used on aircraft diesel engines.