What's it for?

Are you talking about what they hang the ranch's sign from?

That's ridiculously self explanatory.

Farm gates here have two uprights and a pole across, high enough to let vehicles and equipment through, that carries the hot wire from one side to the other, with the gate itself isolated from the fencer.

Keeps the gate posts from falling into the middle causing the gates to drag the ground...........

H bracing, and N bracing, do that job, and probably better.

Thanks , that's a corner post brace. I mean high to drive a truck under.

Yeah, absolutely can be a corner post brace when used on both sides of the corner, which around here are mostly N braces, but an H brace on the hinge side of a gate does a better job of preventing a sagging gate than an over the gate pole. An H brace, or an N brace, can take a LOT more gate weight. Which is what my comment was directed at if you notice I was responding to Muffin, not at answering your OP....I did that earlier to the best of my knowledge/experience with farm fence and gates around here.

A gate overhead pole, beam, or truss can have so many purposes with each possibly only site specific. The most common might be signage, support in soft shallow soil or solid rock, support for wires cables, phone line, decorative, power, lighting, water line, wildlife roost, mailbox swing support, noose for offenders, and whatever may come to mind at your site.

Noose for offenders.

I like that idea.

Wonder how many horse thieves swung at the gate back in the day.......

And left for a few days as a deterrent for their compadres? Like when we were boys we used to hang dead crows and/or groundhogs on the fence thinking it would discourage other varmints. 😬