Light everyday cutting where a pocket is unavailable or inconvenient. A neck knife is a small fixed blade, usually under three inches, worn on a cord under a shirt in a friction-fit sheath. It is a backup and a convenience carry rather than a primary working knife.
It is a fixed blade with no pivot to clog and no lock to fail, in a package small enough to disappear under a shirt. For anglers, kayakers, and anyone whose pockets are full or wet, that combination is genuinely useful.
Draw is fast and consistent, because the knife is always in the same place and orientation.
Small handles mean limited leverage and limited comfort for sustained cutting. A neck knife is for opening, trimming and cutting cordage, not for camp chores.
Comfort over a full day varies a lot by person and by sheath. See is a neck knife comfortable.
The sheath must retain the knife when inverted and shaken. A breakaway cord is worth having if you work near machinery.
Concealed fixed-blade carry is treated differently in different jurisdictions. Check your own rules before wearing one out of the house.