Cyclists ride in such a wide range of weather conditions and temperatures and at different effort levels that a range of cycling kit is needed most of the time to help us feel most comfortable. Moving at speed also creates a lot of wind or wind chill which can also affect our core temperatures.
A gilet can do a great job of keeping your torso and core warm and comfortable whilst not adding too much bulk or when a thicker, heavier jacket might be a bit too much. The ability to carry a gilet easily in jersey pockets means they can really help us keep comfortable in changing weather.