The bride looked radiant, a fairy tale princess.
The bride looked radiant — fairy tale princess.
She'd known him all her life, a great friend.
She'd known him all her life — great friend.
On these types of structures the phrase won't tag on the end of the sentence without a determiner as is the case with most supplemental phrases. Without the determiner (a,the) you need a dash or a colon.
What is the determiner doing to the phrase so that it can take the comma compared to a phrase that hasn't and needs a em-dash or stronger punctuation?