It's a figure of speech.
Dressed to the Nines Meaning (according to The Phrase Finder)
"Dressed flamboyantly or smartly."
Personally, it feels good to "dress for dinner." It doesn't mean a full formal, unless that is what you want. I am a jeans girls myself and my husband is most comfortable in hunting apparel but it's nice & it's special to dress up a bit for dinner. Everyone feels good when they look good.
