I once asked a friend, during a nosy discussion of our respective sex lives, under what conditions he tends to most enjoy sex. I figured he might talk about particular sex acts, positions, or roles – but instead, he said: he strongly prefers sex where everyone involved is completely naked. He'd had awkward experiences before where this was not the case, and so now he felt most relaxed, most free, and most receptive to pleasure when he and his partner both disrobed completely beforehand.
I must admit I was surprised by this, because nudity has never been a particularly important aspect of sex for me. A lot of times when I masturbate or have sex, I only remove the articles of clothing that are directly blocking my path to the relevant bits, and sometimes not even those – a wand vibrator works perfectly well through pants and underwear, after all. If a higher proportion of clothes come off, it tends to be because the person I'm fucking initiates the removal of clothing – or because I'm overheated and need to cool down.
I just don't really care about being naked, and sometimes it's actually sexier if I'm not. Maybe my positive associations with half-clothed sex come from the days when, as a teenager, I'd keep my outfit partly on during sex in case a family member were to open my bedroom door (which lacked a lock). It almost never happened, but each time it did, I was beyond grateful to be able to flip my skirt down and instantly conceal any evidence of tomfoolery (besides maybe my blushing face).
I think a common reason people may not want to get naked during sex is some flavor of body image issues, whether they be dysmorphia, dysphoria, or just the toxic soup of self-judgment we're all fed all the time by tabloid articles and social media. This mostly isn't a factor for me currently, because I know and trust that my spouse adores my body, but it has sometimes come up for me in the past, especially when sleeping with newer or short-lived partners who had not yet expressed liking how my body looks. I want to be able to relax and enjoy myself during sex, and sometimes it's easier to do that if I keep my dress on, so I don't have to wonder what this new person thinks about my stomach or boobs.