There were quite a few of these in my later teen years through early 30s.
In my view, there were a few of likely reasons.
1. Most/many witnesses never participated in typical "rites of passage"-- school dances, junior/senior proms, homecoming dances, etc. These formals allow them to experience something similar.
2. They tend to be a good opportunity to expand your dating pool. Look your best and meet prospective boyfriends/girlfriends.
3. My personal feeling is JWs dressing more formally than the average person on the street for events like church, these "formals", etc., is rooted in an inherent inferiority complex. Deep in their brains, they realize their general lack of college education excludes them from many of the career choices "The World" sees as prestigious, or at least upper middle class. As a result, they try (albeit misguided) to "dress the part"-- silk pocket handkerchiefs (please, that is so 1990s Men's Wearhouse...along with dark colored dress shirts...pretty sure JWs are the only ones who still buy/wear those), pinkie signet rings, etc. Attending a formal, black tie affair is just another manifestation of that, for many. Of course, hosting said Black Tie Affair at the local public rental hall instead of the St. Regis, Ritz, private club, rather takes away the grandeur.
4. As @just fine says, it's "just fun to get dressed up"