Maybe they were mainly older man and Kids, that were not fanatic/or well trained enough and not very fitted for the Job as a soldier? But thats only my guess...
Depends on the division.
The German replacement system worked rather differently to for example the US-one.
Every Division had a "Feldersatz-Battalion" in which fresh replacements were gathered and got a last training before going to their frontline units. These men usually never saw combat but should be physically as fit as the time allows and their respective division needs them (so young and fit men were usually found in Panzer- and Panzergrenadier Divisions while men who were "unfit for service" were usually found in e.g. a Static Division).
Germany differentiated between the Feldheer (the army at the front) and the Ersatzheer (units at home, organizing the training and replacements for the Feldheer).
Now the main difference is that every Division of the Feldheer was related to a Regiment within the Ersatzheer.
These Regiments were responsible to train the Replacements at home, take care of soldiers on leave and taking care of wounded for their respective Divisions (which sent trainers back to the training regiment). At the same time these Regiments were usually recruted out of a certain area within Germany, as for example South-Bavaria, East-Prussia, ...
This system had the huge advantage that the men within a division often came from the same area of Germany and Wounded men were whenever possible sent back to their same unit they left for whatever reason.
On the other hand the German emphasize on maintainig a kind of "esprit de corps" caused them to rather fight with understrength units than to refill them on the frontline.
So a man who was drafted into the army was sent to a Training-Regiment that trained this man directly for the division he will serve in, with other men out of the area where he lived.
After his training is finished he gets sent to the Feldersatz-Battalion were the Division will finish his training before sending him to his frontline-unit in which he will serve for the rest of the war.
So "Ersatz-Units" are pretty much soldiers who just finished basic-training at best and are thrown into combat.
