Shrug jackets can help a woman feel a little bit warmer when wearing a strapless dress by covering her shoulders, which tend to get cold easily. It also covers a bit of the chest, which may make some women more comfortable. But of course it also looks great, and can be used just for its look and style. A silver colored bolero shrug would go well with a grey, white or (of course) silver dress, but since silver reflects light, it could go with just about any color.
Here’s a nice example – I love the “poofy” sleeves:
Silver bolero jackets for evening dresses pictured: Luxury Divas Short Sleeve Dressy Satin Bolero Shrug Jacket