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Well one reason that people get into Visual Novels rather than their anime/manga counterparts is because of the amount of cut content. It's pretty much the same reason I'm hesitant to get into a 12 episode anime that has 12 volumes worth of manga, or most short-ish anime that associated with Light Novels. I feel like I'm missing out or will be given a half-assed, unsatisfactory original ending that basically tells me "go buy the manga". When I was younger, I used to watch anime of manga I read and vice versa all the time, but nowadays I pick and stick to one, and generally I choose the anime as physical manga are becoming harder for me to come by affordably.
In the case of Visual Novels though, it's almost inevitable that a good amount of content and almost all choice will be gutted (with a small bit of the latter often being released in the way of OVAs or, rarely, completely different adaptations). As a result, it's understandable that an anime and a VN would be attractive choices. Generally people will play the former to get the full experience and then watch the latter to see how it's adapted. And there's nothing wrong with only watching the anime, since it's basically the same as watching only the anime of a manga original (which is pretty much all I do nowadays). And then there are those that just want a text-based experience, one that they want to read rather than watch. Heck, maybe they don't like anime at all and have a thing for VNs (and possibly manga). In the end, they're just alternative media.
Though personally, my favorite VNs are the ones that engage the player and generally don't gain adaptations, such as 999, Virtue's Last Reward, and the Ace Attorney series. Though then you get into the gray area of what qualifies as a game and what doesn't in terms of VN.