Good question. For me, it has just been experience.
I've found that the preview window tends to be glitchy. I don't know what causes it, but I've had obs crash on me multiple times when using that interface.
Also, the virtual cam allows you to output at a given resolution but if you want to do the same with the preview window, you need to scale it up which just beats more desktop space and may also just eat more resources.
Finally, it is just more direct of a connection. I would rather be able to get a feed directly out of a program rather than have to go the window-capture route as that just add an additional layer of possible bugs, jitter, and fail.
I've found that the preview window tends to be glitchy. I don't know what causes it, but I've had obs crash on me multiple times when using that interface.
Also, the virtual cam allows you to output at a given resolution but if you want to do the same with the preview window, you need to scale it up which just beats more desktop space and may also just eat more resources.
Finally, it is just more direct of a connection. I would rather be able to get a feed directly out of a program rather than have to go the window-capture route as that just add an additional layer of possible bugs, jitter, and fail.