The idea is that silk flutters in the breeze, so it’s good for banners and standards.
This is good for decorating churches and processions, especially since it’s totally different from felt. These are actually “serti dyed,” not painted. This makes the colors last a lot longer. It’s also okay for outdoor use in the rain.
I suspect you could also use this “serti dying” technique to make nice vestment patterns. Silk vestment fabric would have to be a lot thicker than silk scarves, but I bet you could applique this kind of “banner” to a plain, solid-colored vestment. This would also permit having different designs for different occasions, I suppose, if you made the banner fastenings reusable. You’d have to do something about thin silk’s flutterability.