Speaker level input means that the input is ready for amped signal. Like you can plug in the speaker wire that are already amped from the stock or aftermarket stereo in this input.
Preamp inputs are inputs that are ready for signal that in not amped by a stock or aftermarket stereo. You will get cleaner sound using preamp inputs.
A converter like this will take the amped signal and run it through capacitors to quiet the signal so it doesn't get to the preamp inputs amped. I have done it this way before and the signal is way too loud for the preamp input and it causes distortion.