Matrix Games Forums

Are these forces also used to provide replacements to front line units or are they intended to used to 'fill in' newly created units?

Both.
Filling up worn out or new units goes automatically at the start of the turn.

If you have spare troops in OKH you might consider building new units, but as Germans that hardly happens as you can't even keep up replenishing excisting units.

Exception is for instance if you play several cards of 1 kind(ie artillery) then you have so much artillery in OKH you can create new units and let them be filled up.