Loidrial:
I got it, unit fillers are just deployed because you dont have enough of that unit so you improvise with something, ok cool.
I have enough models so i dont need any of those things, but after i Painted 20 dwarves, thinking of paint another 20 of them makes me struggle a bit.
So i thought “what if i put a couple of ogres in the unit?” then i was like:
How do they work? it will take the place of 4 dwarves, but how can I do it with wounds and everything? Because it will soak 2x2 bases in the normal unit.
I never figured it out how people move wht unit after starting to having some loss
Zuh-Khinie:
For example: after taking some losses on the back rank, you have to take 3 more on that unit… you just take the ogre out, and place one dwarf back.
Simple as that really 
Bloodbeard:
Yup. Just shuffle around the models. I have an ogre in my fireglaive unit. At some point when the unit has lost a lot of rank n file troops I take out the ogre (and perhaps place some dead models back in his place). Say the unit started 20 strong = 16 models and my cool looking ogre (counting as 4).
I’ve taken some damage and are now on 14 troops = 10 models and the ogre (still counting as four regular joes).
Now 2 more models die, I take out the ogre (counting for 4 models) - that doesn’t add up. So I just place 2 of the earlier dead models in the unit.
I use unitfillers because it looks good. Never mind the logic or anything like that. I have a captured slayer, pulled on a wagen, standing on top of a pile of firewood. Obviously he can’t fight, but he counts as 4 Infernal Guard - and he looks cool mixed into the unit.