The thing is, a lot of mixers attenuate the signal at the line inputs and route that through the preamps, where the signal is re-amplified. Mackie mixers do this, as do most if not all of the home studio/project mixers of the last ten years or so.
So while you are plugging your gear to a line input, the signal is still going through the mixer preamp (after being brought down to mic level). But for practical purposes, mics are connected to the XLR inputs, and synths etc. to the line inputs.