Because 24hz movies are blurry as hell, and on a projector it gets tiresome watching blur-fests where as soon as thing start moving things get nasty.
FI is especially important in 3D as it makes everything seem more solid and the 3D effect is MUCH better.
It's important for projectors to have it built-in, because as much as I love SVP, it's buggy and crashy as hell, and anyway since HDMI 1.4a (which is what most projectors accept), cannot handle 1080p/60/3D (frame packed Bluray 3D signals increased from 24hz to 60hz at full resolution), it means you have to rip your own 3D Blurays to SBS or O/U formats, do the interpolation using SVP. So you trade off 1/2 resolution for 2+ times the frames, which is kind of a step forward and a step back at the same time (resolution matters on large projected images).
Having the FI in the projector is a huge benefit in terms of convenience, and ability to play back both 2D and 3D Bluray sources (like from an Xbox One or PS4 or a lower end PC), since SVP takes a lot of GPU horsepower and that means I can't use my cheaper PC to play back shows. In short, it sucks. There's also the problem of anarmorphic stretch mode not working on frame packed 3D formats, which AFAIK
none of the BenQ projectors can do. At least not the low end ones. Which means I can't use my anamorphic lens with 3D movies, and again, I have to rip them first. And I hate doing that because it's a huge pain in the rear. I pay for my movies and all this encryption nonsense makes my life harder. It's actually simpler for me to buy the Bluray, then download the 3D SBS rip from an illegal source, than it is for me to do all that work myself. And half the time the ripping software doesn't even work.