The variety of responses to this query has convinced me that one could probably reliably start an entire academic field around it complete with multiple schools of thought, ranging from the impregnation purists to the belly fetishists to the birth lovers.
I personally am one of those people that love the whole nine yards, from impregnation all the way to birth. I love how a woman's breasts grow larger to prepare for breastfeeding, how their nipples turn darker, how the stomach grows overtime, how they become veiny. I love it when games incorporate the experience and struggle of pregnancy since it adds a layer of finality and permanence to your actions. This is all you. You did this. And it is great. I love it when there are X-ray visuals of the uterus and you can see the ovum or ova just waiting to be fertilised and implanted, when they develop into fetuses and you can see them change with each passing trimester. Even birth, from the painful to the unrealistic pleasurable depictions, fascinate me. Do they love it? Do they hate it? Are they anxious about it? Excited? Do they go out of their way to consume forced ovulation drugs and all sorts of fertility drugs to pump out multiple at a time?
To most people, the fetish is a primal one, a base one. So ingrained in our psyche that it might as well be just part of the human psychology, not a fetish but a life process. But to me, what defines it as a fetish and to myself is the active seeking out of it. The desire of that life-changing outcome that makes it forbidden to enact freely unless the other party is consenting or if you're already married. The S&M enthusiast might be judged for their deviances but the pregnancy fetishist, regardless of the impregnator or the impregnated, must think about long-term consequences of their actions since a child is a huge responsibility and getting repeated abortions may have damaging effects to health. It can ruin entire households if one party just so happens to be cheating on their SO... or make it devillishly enticing as the unsuspecting spouse will never know that they're taking care of your progeny.
It can even be more deviant in nature when combined with other forbidden kinks like incest, loli, shota or bestiality since it's almost like a slap in the face of conventional societal norms. Imagining that the gravid co-worker in your office isn't carrying the child from a one-night stand but from a stray dog, that a relative is unknowingly carrying the child of her cousin due to being raped in their sleep, and so on and so forth.
I could go on but this is essentially why I am into impregnation and pregnancy as a fetish, and also why most games that use the pregnancy tag annoy me so much.
The art looks good, the sex scenes are amazing, but the pregnancy is ending only. Or the game does have a pregnancy mechanic but the woman just instantly bloats up and just deflates a few frames later with a bean counter indicating number of births telling you that it has happened.
These are also the reasons why games like Violated Heroine, Vitamin Quest, Drop Factory and Hazumi and the Pregnation are often the top picks for most impreg/preg fetishists since the developers understand what appeals to those that like the fetish instead of merely plopping on different renders and calling it a day. It is also why you more often than not see quite a few people clamoring for the inclusion of pregnancy in early access games because while there are a wide selection of games with the tag to choose from, games that actually make the kink the core focus are pretty limited by comparison.
