Ghoszo said:

I mean, why do they even exist? all people do is avoid them...

Yes, almost all of the time we avoid them. 

And they exist so that we can appreciate the moment when we get % based equpment instead of flat stat. 

As someone was saying on Discord the other way, in theory flat stats could be useful for providing a larger boost than percentages for players who are inherently weak on that point. E.g. for offense champions with fairly low base defense, a flat defense piece with a high enough number could be superior to a def percentage piece. But in reality, the flat stats numbers are too low for that to work out, except at low champion levels.