How exactly does poison work?

Sorry for the probably dumb question, just can't find it clarified in a way I can understand. Probably just not searching right. But, my question is how do you determine and scale poison damage? I saw that it has like a 28 damage over 3 seconds default. My question comes from me looking at Heartseeker's Blightshot, which gives it chance to poison. If I wanted to do that, do I try to focus on scaling poison somehow, or should I still be scaling physical damage for Heartseeker?

Or is this even worth it? I saw, though possibly dated, that a lot of people were saying poison isn't good :(