I imagine it’s used/recommended since glucosamine is naturally found in shellfish coverings and has anti-inflammatory properties and is thought (proved?) to support cartilage growth.
My stepdad owns a company that makes a human supplement for osteoarthritis which contains green-lipped mussel from New Zealand and I was looking into it for both myself and my horse at one point. I did some research on shellfish sources for cartilage support during that time, and there’s lots of positive reviews from humans about its effects, but unfortunately the amount that you would have to feed a 1000lb + horse to get any therapeutic effect was a major deterrent!
I personally feel the cost can go elsewhere (especially for hooves/hair) into a complete vitamin and mineral source and basic flax has served me well for the omega 3:6 ratio for years. That being said, I’m not afraid to try different supplements as long as the ingredients are readily available for viewing and there’s some research done on them, but every time I give up on them and just keep plugging along with the vit/min and flax 