"Cool Vest" seems useless?

Decreasing insulation will make the dupe more susceptible to cold AND to heat. They will heat up and cool down quicker based on their surroundings - something you never want, right?

Increasing insulation will make them less susceptible in both cases. So when travelling though hot and cold zones, it helps in both cases.

So why would you ever use the Cool Vest? Or have I misunderstood something?

The only scenario in which the Cool Vest seems useful is for micromanaging to help dupes heat up/cool off more quickly after getting too cold/hot, but not for permanent usage.