Its for holding small cylindrical objects or, possibly threaded items like screws, in a vice so you can work on the end or head.

Usually threaded in appropriate sizes when used for screws or other threaded things to minimise risk of damage.

Basically slide or screw the part to be worked on in as appropriate and clamp up the vice. The slit lets it close up enough to grip hard but not so much as to damage the object,

Clive

Likely a pipe support for making flanges on the ends. Typical use would be flaring automotive brake pipe ends.

These are ones used at the cheaper end of the flaring kits available.

LOL. Yes I have the one I made too, still lying in the toolbox where it was put on completion. Never been out that I remember. Seems like soft jaws in the vice do the job every time.

Flaring tools are similar but usually come apart into two pieces for removal and have chamfered holes to make the flares.