To be non-interacting with anything... and that makes them rather foreverish.
The problem is that the structure of the PFAS (a fluorocarbon chain with an OH at the end) looks a LOT like a hydrocarbon chain with an OH on the end... that's a saturated fatty acid which is crucial for body metabolism.
So yes, it had one job - and it did that job well. It also looks like something that has another job... and does that job really poorly when substituted.
The problem is that the structure of the PFAS (a fluorocarbon chain with an OH at the end) looks a LOT like a hydrocarbon chain with an OH on the end... that's a saturated fatty acid which is crucial for body metabolism.
Compare the structure of https://en.wikipedia.org/wiki/Myristic_acid and https://en.wikipedia.org/wiki/Per-_and_polyfluoroalkyl_subst...
This allows the body to accidentally substitute a PFAS (which does nothing) for a fatty acid (which does something - and often important things).
https://www.vox.com/2022/8/25/23318667/pfas-forever-chemical...
So yes, it had one job - and it did that job well. It also looks like something that has another job... and does that job really poorly when substituted.