You'd be surprised how much warmer you might be if your core is dry. Especially in warmer rain, just keeping most of you dry goes a long way. It can also come in handy for layering strategy. In some parts of the world, a softshell cycling jacket might be plenty of warmth. The only problem with that type of jacket is that sustained rain will eventually come through. If it’s rare for you to spend time in sustained rain then adding a weather resistant gilet gives you an option for adding warmth to a summer jersey as well as bulking up your winter softshell for the rare times you need it.