A CTA turns attention into a result. Without one, a good post is enjoyed and forgotten. It should match the goal of the post: a sales post asks for a booking, a tips post for a save.
The best CTAs are concrete and easy: they name the action, the place and, when it helps, the reason. “Link in bio” alone is weaker than “Book a table for Sunday via the link in our bio”.
Example
“Save this for your next grocery run” on a recipe post, or “Book a trial week, link in bio” on a gym's post.