It’s not for low light. Smaller apertures give greater DOF (depth of field). Large apertures give shallow DOF (great for portraits to soften the background).
However, with a range from f1.5-f2.4, there’s not much range at all (a typical SLR lens would have a much wider range, like f2.8-f22, for example).
What you said about DoF is all true for cameras with larger senors, like a SLR or mirror-less micro 4/3, but less so for a camera phone.
Camera phone have much smaller sensors, so have lenses with focal lengths of only a few millimeters. I think the iPhone's lenses are 4mm and 6.6mm. Even at f/1.5 or f/1.8 these lenses have a DoF that extends to infinity, unless focuses on a object very close to the camera (I'd guess, less a foot away). The fact that camera phones can't blur the background optically like an SLR, is the reason Apple, Samsung, Google etc, have portrait modes that blur the background computationally.
Most camera phones have a fix aperture lens and alter ISO and exposure time, depending on the light. It seems Samsung have a lens that can be set to f/1.5 or f/2.4. For low light situations f/1.5 will let in as much light was possible, I am just guessing, but f/2.4 might be slightly sharper and have larger DoF at very short focal distances.
As you said a SLR lens will topically range from about f/2.8 to f/22, depending on the focal length. Camera phone lenses don't go much passed about f/2.8, again due to the small sensor. F-stop is a measure of the aperture as a faction of the focal length of the lens, a 4mm lens at f/16 would mean the diameter of the aperture would be 0.25mm so small that the camera is Diffraction-limited and the image becomes less shape. SLR become diffraction-limited at around f/8 camera phones it is about f/2.8, so their lens don't have apertures smaller than this.
saving for a wedding is more important.
