Agility heros: Dmg vs Agility

I wouldnt call myself "new" exactly but close enough... I have been wondering why all the builds for agility heros seem to suggest getting damage based items. I dont understand why you would get damage over agility when agility heros clearly get damage, armor, and attack speed all from agility. So am i missing something? or are the builds just wrong?

The builds aren't wrong, it's just that most agility carries need various other things first as they tend to lack HP to survive the early-mid game magic damage - hence why Black King Bar is pretty popular. Lifesteal goes a long way. Going straight for an agility heavy item tends to leave you as a bit of glass cannon.

There's also not that many options - Diffusal Blade is pretty popular on the right heroes, Yasha into Manta Style etc. Butterfly is very much late game, and Ethereal Blade is very situational. Ring of Aquila is pretty handy if you need some extra mana, but is a fairly cheap item.

It also depends on the passives they have - e.g. Phantom Assassin wants to stack damage in the hope of big crits, and straight damage items tend to offer more "bang per buck". Basically agility isn't worth that much early game, but becomes very important later.

Agility heroes generally have a high agility growth, so you don't have to invest a lot in agility items to have a good amount of attack speed and armor. Building damage is the better choice here, since your attack speed is already quite high and agility items are quite expensive for what they give.

Alright i guess i didnt think of it as a bang per buck perspective...but for phantom assassin does the damage added from agility items add to the critical chance damage? or is that just for straight damage

It adds to the crit damage.

Yeah, in fact there are generally more things that stack with base damage/primary stat than with other stuff - e.g. Vengeance Aura and Enchant Totem

Daedalus and Butterfly on all the heroes.