Do you mean like a shelf or platform, as opposed to a full second level? Syrians love having another level/shelf - it gives them something to sit under to feel secure and they also seem to like sitting on a shelf to have a wash (maybe because it's stable so they don't topple over when grooming
). It's also a good place to put any hard items that might sink in the substrate or get tunnelled under - eg ceramic items, a food bowl etc.