This is something you can say even to a stranger who is doing physical labor out on the street and the other party will thank you for saying it. It is a means of empathising with the people doing a hard task; you tell them that you hope in the end the task gets completed without much difficulty. In some contexts it is even rude NOT to say this to a person (even to a stranger) if you observe them busting their ass working on something (fixing a car, digging a hole, carrying heavy stuff around etc.) If you are interrupting someone working on something by themselves, you say "hi" by saying "kolay gelsin", a kind and expected thing to do. If the worker resents doing the job, and if you are close friends with them, instead of thanking you for saying "Kolay gelsin", they might playfully respond with "Kolaysa başına gelsin" which literally means "If you think this can be easy, I hope you end up as the one having to do it by yourself". Again, it is very rude even aggressive to respond this way if you two are not close friends or close family. Many Turkish people have hard time in countries where this phrase does not translate. It is very uncomfortable for us to observe someone working hard on some stuff and not having anything customary to offer as a verbal sign of empathy. We feel like not saying "Kolay gelsin" is like not caring about the possible difficulty the person doing the work is facing.