The course of action will proceed in an overly linear, overly predictable fashion. A smooth road or easy path can be wonderful in real life, but actually, such a situation is rare. Therefore, such a course, if depicted in a story, is rather unrealistic. And that's the cardinal sin for storytellers. In addition, a story where the characters are confronted with too few obstacles is unrevealing.
Obstacles test a character's resourcefulness and resolve.
Sometimes they can even reveal modes of thinking, priorities, and values. Finally, obstacles add uncertainty, provoke curiosity, and can create or augment suspense and tension. Events or situations. Furthermore, obstacles can be short, intermediate, or long-term in duration. Some obstacles pop up quickly and are disposed of, overcome, or dodged just as quickly.