What I like about it with actual partners or playmates, IRL, is physically the tightness of it, but far more mentally the feeling that no part of her body is off-limits or out of bounds to me, that all of her holes are available for me to use as I see fit. It’s interesting to me that in every romantic or sexual relationship I’ve ever been in (not talking one-offs here, just repeat customers) the women have been open to it, and some have actually requested it, fairly early on. It makes me wonder if that’s a result of the type of woman I tend to pull, or if lots more women are into than will say so publicly, or if it’s something women will do with one partner but not another, etc.
In rape fantasy fiction, by contrast, I think it adds a dimension because it’s an extra level of violation, likely painful and humiliating, potentially something she’s never done before and wouldn’t be ok with in a voluntary encounter. Also allows more opportunity for more simultaneous attackers.
On the gangbang thing, it’s similar. One man simply cannot use a woman to her full capacity. Multiple guys allows more violation and humiliation in fiction, or more stimulation and fun if she’s into it IRL. For myself I’m not a huge fan IRL, but in fiction it’s great.