But if it's weight triggered on both ends, what does it actually stop?

Cows

But cows are heavy, surely they'd lift the gate

See those little slats? Cows recognize early on not to step on them or their legs go through.

If you ever seen one of these on a rural road thats why they are there. To keep cows in one area. They don't have the coordination to cross it. Sometimes you can just paint the lines on the road if cows are used to these cattle guards and they won't attempt to cross.

That gate up there is very heavy. A cow would have to step on the proper slats with all of its weight for it to open, once it moves it comes back down. 99% unlikely for a cow to get through it

Could not a cow easily walk through this gaping hole in the fence?

Yeah but thats not important