Brassica building?

Yes they are for brassicas in the main but make great shelter for all veg. we have fix some tube into the ground and slid the poly pipe over the tube and netting over the top with large stones to hold down the netting seems to be working so far.

Some of the structures you've seen could be fruit cages - they're going to be permanent (as the plants don't move!) and often need to be reasonably tall (depending on the plants), so some people make them big enough that they can get inside and pick the fruit without removing the netting.