The very useful thing here is that in thick mist like that the cairn can confirm for you exactly where you are. And cairns are quite often placed at intervals going down from the summit, invaluable in bad weather when it's really easy to lose the trail. (Like this, not my photo)

Interestingly, some hiking sites are now saying that this is environmentally damaging, and that hikers should destroy rock cairns when they see them.
I profoundly disagree.

I posted
here back in September about a local incident when two hikers in bad visibility got disoriented on Quitahuayco (the highest mountain in the area), went down the wrong side of the thing, and never corrected their mistake. They were eventually found 5 days later by a search team, and one of them died of exhaustion and exposure. If there had been rock cairns, even small ones, marking the correct and safe way down, this would never have happened.