The main reason is the sheer number of free and fairly reasonably priced ready-made 3D assets for Daz. Characters, outfits, locations, special effects, custom shaders, pose packs, this all translates to less time required to build and set up scenes. Also, the Daz Studio interface is fairly easy to learn, and there are a bunch of tutorials on the net for Daz Studio these days.
The number of free assets that are ready-made for Daz Studio or that can be imported (.obj ans such) is pretty extensive these days. Plus Daz Studio is free.
Blender is also free, but it can't match the Daz Studio library. A lot of that library is 'for sale' assets of course, but you can buy as you go with Daz, and/or go the buccaneer route. Between Daz3D, Renderosity, Hivewire, ShareCG, PoserDazFreebies, Renderotica... yeah there are a number of sites selling assets for Daz Studio these days.
Other programs may be stronger capability-wise, but it's the sheer size of the Daz product library that makes it more attractive.
For game designers, 3d comic artists, and VN types, yeah it's a very useful tool. Of course, if you can draw decently, that is still a bit more popular these days, at least according to the top Patreon adult games list.