In contrast to Rich i find gilets very useful. Once I got one I wondered how I'd lived without it.
Why? You can wear them with a short sleeved jersey to keep the chill off. They also offer protection from the wind and rain without the clamminess of long sleeved waterproofs. When it gets colder they can be another layer over your winter jersey to keep the wind chill off your chest. All this AND when they are not needed, they can be stuffed into your back pocket - ready to re-emerge when needed.
Why? You can wear them with a short sleeved jersey to keep the chill off. They also offer protection from the wind and rain without the clamminess of long sleeved waterproofs. When it gets colder they can be another layer over your winter jersey to keep the wind chill off your chest. All this AND when they are not needed, they can be stuffed into your back pocket - ready to re-emerge when needed.