I run them when I'm having some sort of issue, like recently I was dealing with a dino explosion so I was running a sock and turkey baster to blow the snot off of the rocks. I was swapping out my socks every other day during that period.
When I was having issues with low nitrates, I left a sock on and didn't change it for over two weeks.
If I'm going to do a water change, I'll throw one on, blast the rocks and bottom with the turkey baster and turn my MP40s on nutrient export mode to get all sorts of detritus stirred up and removed. I'll leave it on for a day after the water change and then go sockless.
There is no hard and fast reason to use or not use a filter sock, they can be entirely situational as I listed above.