How does +x% increase rarity work in depth?

I never played POE before how does this stat effect loottables? I understand the difference in item rarity (normal, magic, rare, unique). I understand the identification tiers, dropping lower tiers of modifiers so high tiers are more common. I understand that loot overall gets better, high tiers or high value currency drops. I want to know how it is doing this. Is it like enough increase in rarity enables you to find better items you couldn't find before? Does it increase the percentages of high tier drops or lower the drops directly? Does it improve loot in the way that the identification tiers work? so high stat means low value items drop out of the loottable?

Maybe its a little too early to go into this in depth but there could be information from POE1. What I'm trying to answer for myself is, if it is worth it to go as high as possible with this stat, should I have a certain amount of it or is it not worth it at all?

Im loosing myself right now googling and reading for hours. I hope someone can help :)
Thank you!