During WWII, it seems the Germans were particularly interested in building self propelled Artillery that had the onboard facility to offload its artillery piece and set the weapon up as a stationary ground based artillery without using sort of depot service. Several designs reached prototype stage, although losing the war stopped any production plans.

It would seem this feature is not very useful during mobile warfare, where the premium would be on self propelled artillery keeping up with tanks and delivering fire support promptly in a fluid battle situation,

It would also require the self propelled gun chassis to be much bigger than otherwise necessary. So this 105mm self propelled gun with autonomous off load capability had to use the same chassis as 150mm howitzer without offload capability.

Why did the Germans go for this kind of extra complexity and cost which seem to offer little added value much impose much added cost?

Surely where stationary artillery is needed, a real towed artillery piece would be both adaquate and much more economical?

I am no expert but I think the idea was for a Panzer Division to not only have tanks but other arms like artillery incorporated and could keep up with the tanks. I don't think towed artillery was capable of that. The guns would bounce about being towed at speed cross country.

What is the advantage of self propelled artillery accompanying panzer division being able to offload the artillery piece?

If anything that makes the panzer division less mobile by requiring larger and therefore more fuel hungry chassis for the self propelled artillery.

If the artillery is to be emplaced the vehicle can be used to ferry ammunition (waffentraeger). Not all mobile artillery could off load their guns like that, and needed separate munitions carriers so using up two chassis against one.

The chassis has make 2 round trips, one to bring the gun up and off load it, the second to bring up the ammunition. While the chassis is off getting ammunition, the gun would be stuck and can't be moved should the enemy counterattack.

So I am not sure if using the chassis as ammunition transport on e the gun has been offloaded really works. Having a separate self propelled gun and an ammunition carrier means all the a munitions are immediately available, and the gun can instantly move whenever needed, and doesn't have to wait for the chassis to come back.

I believe the thinking behind this is that the motorized chassis is a complex and delicate (not to mention expensive) piece of equipment and should not be exposed to overmuch danger such as counter-battery fire. A second point was that a disabled chassis for whatever reason also meant the gun was out of action.

The waffentraeger off loads it's gun AND ready use ammunition carried on it. The carrier is free to fetch more ammo or has been pointed out, find cover. The mobile artillery piece either has to have a separate munitions carrier or leave the field to resupply.

The old guard of the artillery always viewed the gun as paramount. If the horses or the tractor were lost, they could be replaced and it was still possible to save the gun. It would appear that they weren't comfortable with the idea that a valuable gun could be lost just because of a mechanical issue with the chassis. The weapons carrier designs were needlessly complex--it was wrongheaded, and nobody builds self-propelled artillery with that feature anymore.