Traveling longer distances one doesn't get the choice of nice warm weather every day. If you daysail your boat or only go out on nice weekends that's great, but some people like to be warm and dry when they travel.
When we took our boat to the Bahamas from the Chesapeake in November, there were few warm dry days. We met may purists wearing full exposure suits obviously not enjoying life, while we were in sweatshirts inside our enclosure.
Don't travel without one.