Spring and Fall are when I seem to pull out my odd vests. A vest provides another way to vary the look of a jacket and trousers. It also adds a layer of warmth that extends the wearability of a light-weight jacket into cool weather, and a mid-weight into cold weather.
In the photo, I'm wearing a linen odd vest with a fresco odd jacket. The day is windy on the coast and that wind whistles right through the fresco. In combination the two are perfect for the conditions.
As I've written before, a vest requires trousers that will stay up without a belt as a visible belt buckle under a vest is a sin comparable to showing bare skin above your socks when you cross your legs. Don't do it.
But do consider the versatility of an odd vest with your odd jacket.