why is it usefull to compact stuff, if the stack balance remains the same?, I mean, I have iron plates stack to 200, I comact 100 in piles of 50, and the stack of the piles is 2, so what's the point in compacting?, I wanted this to gain some storage space

Belt throughput, inserter throughput, and logistic bot throughput are all boosted by stacking items. There are long discussions in the forum thread on why the decision was made to intentionally avoid also boosting storage density and train throughput, but there's no reason a separate mod couldn't add a density multiplier for item storage as well. I might be convinced to add such an option to https://mods.factorio.com/mod/deadlock-experiments if someone asked nicely ;)