even when you have tagged unions, visitors are a useful way to abstract a heterogenous tree traversal from code that processes specific nodes in the tree. e.g. if you have an ast with an `if` node and subnodes `condition`, `if_body`, and `else_body` you could either have the `if node == "if" then call f(subnode) for subnode in [node.condition, node.if_body, node.else_body]` and repeat that for every function `f` that walks the tree, or define a visitor that takes `f` as an argument and keep the knowledge of which subnodes every node has in a single place.