That is a new one for me although I see no reason why they couldn't be used for a fence. We are about to fence in about an acre with electric fence to hold my Daughter's horse and I'm sure we won't be using that type. The amber and green have some real value, especially if you removed then from use. I'm really surprised you found that many in use. Let's see pix if you got 'emOK. Most of those I removed from fences in Western Nebraska. Maybe that was there second life. I have many some are amber and green. Always good to learn.
I believe Botlguy is correct, those are radio antennae insulators, but I have also seen them used as electric fence insulators here in central Cali, mostly the ones shown in your pic on the far right, clear glass, some turning purple from the sunlight, they are pretty cool though....Andy