One of the slots is usually filled with a wedding outfit as either groom or bride. Then there's various kinds of formal, as in formal for a party, or formal for a business meeting. I have a couple of restaurants that have formal outfit requirement, but they wouldn't dress in long formal wedding party dress, more like little black cocktail dress. Then I also like to give sims outfits that look weather appropriate. Then there is the issue of convenience, if you have all slots filled it's easier to find a suitable outfit when you are in a hurry than to try and put something together. I don't have patience to create outfits, but every once in a while the mood strikes me and I create outfits for them. It would be so much easier if they had their own wardrobe like in TS2 and you could select outfits from there, but searching through the whole TS4 catalogue trying to put outfits together is too tiresome.