Ok, so what does Item Quality do and what’s the importance to have a IQ 5% to have a good base item?
On weapons, it increases damage. On all items, it seems to increases the chances of getting perfect (or near perfect) stats on your items when you reroll them. Some of the old heads might have more to say, but I hope this helps. 
Also, with one of the crystals, you can increase it to 25% and eternals come with 50%
Don’t forget the marginal increase in armor too.
You can get 750 AR with 25% quality from 600 AR with the same level 100 item.
Got it, thanks!
It is not marginal. It increases base WD and base AR by the amount shown (up to 25% for normal gear and 50% for eternal gear).