For me having a Springer it is a little different. If I know my dog is going to be retrieving in cold water, if it's close to freezing outside, or sitting for periods of time in cold temps...the vest goes on.
I had a vet tell me 30 years ago that putting a vest on your bird dog will add a year to their life. As the less their body is fighting to stay warm, the better off they will be not only during the hunt, but long term. Made sense, and I've been using the vests in inclement weather since.