What's the point of resettlement "purge"?

[Piotrzeci]

I understand it's supposed to be a way to get rid of pops, without making everyone around you angry, but the thing is; slave market. The pops can just be made into slaves and all sold. It will be quicker and bring a lot of profit. Why let pops go, if they can be sold?

 

[Dainiuskinas]

In my experience slaves don't always sell. I've had this game where I was breeding an organic species as a robot empire to sell them and a lot of the time no one bought them. The way I see it, selling them can take longer than purging.