All wild pokemon can call for re-inforcements, this can create a chain - shiny chance, IV quality and HAs have a higher probability of appearing as a result.
Adrenaline Orb is a consumable item that lets you increase the frequency of this happening.
So it's basically the mechanics behind chaining - except it happens automatically in battles now.
In general this also adds more challenge to capturing wild pokemon as you need to be able to capture your target when it's alone and quickly dispose of its back-up.
So if you want HA Alola-mons for your main team, you can totally do it.
