In this context you could say that CST -> AST is a normalization process. A CST might contain whitespace and comments, an AST almost certainly won't.

An example: in a CST `1 + 0x1 ` might be represented differently than `1 + 1`, but they could be equivalent in the AST. The same could be true for syntax sugar: `let [x,y] = arr;` and `let x = arr[0]; let y = arr[1];` could be the same after AST normalization.

You can see why having just the AST might not be enough for syntax highlighting.

As a side project I've been working on a simple programming language, where I use tree-sitter for the CST, but first normalize it to an AST before I do semantic analysis such as verifying references.