Being a pay player may mean mine is a bit different to everyone else but the way I have it is my normal barracks contains all my units I want to keep (which is usually one of every unit I acquire, I like to collect.) + all my iron and bronze fodder for leveling once I fill this barracks with fodder I bulk lvl one unit up usually from 1-50 or multiple units that need touching up with lvls.

My second barracks is home to all my duplicate silver - black units (No I don't have any double black units but if I did they would be here.) I use them for CC/AWing, CR my main units or for Rainbow Crystals if I get too many of them. For me this Barracks is currently nearly full not sure about if a free player would fill it quickly like this.

My third barracks is home to all my Spirits and Plat armors this is half full as I am filling it again after my last big lvling session.

As I play and Aigis gets more units I may need to rethink my storage solutions as my main barracks might not be able to hold one of every unit plus my bronze, iron fodder.

I am sure there are multiple way to use your storage and having the 2 additional barracks is a huge help so using it for your bronze and iron fodder is a good way to do it as you can hold a lot and as you said bulk lvl any unit you wish to at any given time if you have stored for it.