What a person wears can tell us a lot about them. Along with their physical appearance it's often one of the most important factors in forming a first impression of someone. A talented storyteller will be acutely aware of this, and tailor a character's wardrobe around a specific emotional response that they want to elicit from their audience. This is something that can vary from scene to scene, chapter to chapter, but often a well-constructed character will have a "signature" outfit or look that we identify them with. Bruce Wayne wears a suit. Rocky Balboa has a pair of boxing gloves and shorts. Count Dracula wears a high-collared cape.
A signature outfit can reinforce a character's profession, social status, attitudes, and the setting they find themselves in. Just like how we all have our own personal styles of clothing that reflect certain things about us, the same is true of our characters. The way one of your fictional creations dresses should never be chosen carelessly, as it's one of the most powerful characterisation tools we have to make a first impression with.
For me, clothing is something I rarely describe in great detail in my writing. I've always been a fan of allowing the reader some leeway to paint their own picture in their head (I think this is especially important in erotica, where the physical fantasy of the main characters needs to be as ideal and personal for each reader as possible), but nevertheless, I'm always careful to drop in a few telling hints to guide the audience's perception of my characters.
One of my favourite little clothing tidbits comes from my soon-to-conclude serial Wild Instincts. Most of the characters in this steamy werewolf romp dress similarly; jeans and jackets and boots, the sort of rough and practical clothing that suits a life lived in the wild. The antagonist Cyan, however, is described as wearing a hunting jacket. This is an incredibly minor detail, but I absolutely adore how much it's able to convey about his character through the addition of that single word. He is a hunter, a predator, an alpha male. It gives him a savage and determined edge, and a ruggedness that many of the other characters lack by comparison.
Another intentional clothing choice of mine pops up in His Darkest Desire. While the protagonist Nina is not exactly a secretary in the conventional sense, nor is her love interest Elliot a traditional businessman, I very much wanted to sell the fantasy of the powerful boss dominating his personal assistant. As a result, Nina's choice of fashion often involves crisp blouses and skirts, while Elliot dresses in smart and businesslike attire. Even in several of their bedroom scenes Elliot is described as wearing his classy white shirt, rolling up the sleeves and undoing his top buttons to maintain his air of dominance while allowing a hint of something more wild and passionate to creep in.
Clothing is also something that can be varied from scene to scene to challenge our perceptions of a character. A great example would be the "sexy lady" reveal of a female character often seen in movies, where a previously plain or strictly professional character is seen letting down her hair, putting on a slinky dress, touching up her makeup, and finally descending down a flight of stairs accompanied by some alluring jazz. It's harder to have this same effect in literature, but taking the time to describe how a character's appearance might have changed from one scene to the next is always a handy technique to use when you want your audience to think about someone in a different way.
Perhaps the businessman's suit has a small tear in the shoulder we've never noticed before. Perhaps the golden watch we're so used to seeing around his wrist is missing. He has a coffee stain on the hem of his shirt that's just slightly too big to tuck out of sight beneath his belt, and his tie looks as though it was knotted carelessly and in a hurry.
Of course, none of this is strictly necessary, particular in literature, but it's an important gizmo in your toolbox to be aware of. If you describe a character's clothing, your audience is going to respond to it. Make sure that the response you get is the one you want, and try never to dress your characters haphazardly because you didn't take the time to consider how it might reflect on them.
Signature outfits, wardrobe changes, and first impressions are all powerful tools an author can use to influence subtle characterisation.