Neck knives have been around for a long, long time. Maybe 10s of thousands of years, or even longer.
The main benefit for me is that it makes the blade very accessible. A big negative for them is that you can get hanged if you have an accident and the strap gets caught on something. That is, unless you have a breakaway strap around your neck.
It is actually illegal here in California (what isn't), but I do have one that I wear when working in areas where I'm not likely to meet someone who actually cares. I converted a Frost Mora SwAK to neck carry and I actually like it better that way than on the belt. When busting brush, etc., I stuff it in my shirt to keep it from hanging up.