You are describing several very different conditions. Firstly, night is not "low viz" as you describe it; viz has to do with distance that you can see something and make out what it is, so your clear-water night dives are "narrow viz" (where your light is shining) but not low viz. Low viz can easily happen in the daytime....algal blooms, sand or silt stirred up in the water, runoff from a rainy day...and for really low viz a strobe does not help much at all, but it does help.
Secondly, you can see a strobe at night farther than you can see with your hand-held light.....regardless of whether the viz is good or not. The strobe light only has to go one-way through the water, whereas the light you are carrying has to go two ways, and the strobe is flashing which helps to pick it out at a distance.
Yes, navigation at night without a strobe is usually possible in decent viz, but is always more certain with the strobe. My preference on night dives is a strobe at the boat/exit-turn but natural/compass navigation as a primary or backup tool, since it is more robust when usable but is not as easy (or as accurate!) if the strobe is working. Think of your natural/compass navigation skills and the strobe as like having two lights at night; the redundancy is what keeps you safe.