Both- but there's a big caveat to the actual effectiveness with glucosamine-chondroitin supplements. Glucosamines are a group of compounds that taken individually have widely varying effects. Some are utilized developmentally in building articular cartilage and sinovial joint capsule, but do nothing in adult dogs. Similar compounds have been shown to promote joint regeneration in adults, but are very sensitive to processing (must be cold processed). Further, some compounds that classify as glucosamines have no effect whatsoever, and even of those that have, few have had their effects adequately studied in dogs.
The point is, supplements and foods with glucosamine don't usually tell you which molecular compounds are being used, where they originated, what is the concentration or how they were processed. While I don't think they're detrimental, I think you can end up paying for a lot of snake oil and potentially relying on something that is providing no benefit.