Hi there, resident new mom here weighing in.
While I understand more than most how uncomfortable pregnancy is, I personally like it both as a kink in RP as well as a story element. I think a lot of it, for me, has to do with trust and vulnerability. I'll explain each with as much detail as I can muster, with my experience having been pregnant and given birth still as a very fresh memory.
I like it as a story element because it tends to really throw a wrench in the plans of my characters. Rarely do they actually plan to become pregnant, so they have to learn to trust and rely on their partners. in a time where they are simultaneously the most powerful and yet the most vulnerable they likely will ever be. I rarely play women who aren't powerful in their own right in some way, be it status-wise or in terms of having a strong personality and big plans for the future all the way to them being supernatural creatures with some sort of powers, and I like the level of trust they have to place on their partners to take care of them where they suddenly might not be able to. This is especially fun in a non-fairytale romance type of story, since it leaves people who may not even like each other to grapple with this new aspect to their relationship... which isn't to say that I don't enjoy cozy SOL types of RPs where it's just a couple starting a family, getting married, raising their kids, that sort of thing.
I like it as a kink because of that same sort of vulnerability. In a noncon scenario, it gives the dom a level of control over MC that can't really be compared to. If it's consensual, it's because of the romance and the idealization of it, the fact that MC is carrying the child of the man she loves, a piece of someone she can't do without. It's all very romantic. But, from a purely physical standpoint, and I can attest to this even with the discomforts that come from even the most routine of pregnancies, everything is more swollen, more sensitive, and wetter. Like big, sensitive tits? Like an especially slippery lady? Like voluptuous bodies? It's a no-brainer, really.