A beta reader doesn’t have to be a writer. They evaluate a manuscript strictly from a reader’s perspective. They may earmark an area of concern (like a critique partner) but they may lack the toolkit to help suggest corrections. It’s best to give a beta a middle draft or final draft for evaluation, since this will be closest to the product readers consume and they provide an early view of how a larger group of readers will react.
Critique partners, as fellow writers, are available to help trouble shoot during more phases of a writing process and are able to better evaluate what the concern areas might be along with solutions.