An AST or any intermediate representation of an input language isn't strictly necessary for a translation task. But if you want a human to understand the translation, an IR can go a long way to illustrate the translation process into a human comprehensible form. The transformations of the tree can also provide insight into the various translation activities and alternatives, like translation from one human spoken language into another (text or voice), from one musical style into another (in notation or audio), or source code into executable with multiple optimization steps. Without the AST (IR), the human must remain out of the loop, relegated to being a bystander mystified by the inexplicable magic taking place inside the automated translator.