If students can identify the main idea of a passage, then they should have a good idea about the other sentences in the passage. When the student sees sentences that do not support the main idea of a passage, those sentences do not belong to the paragraph. Those sentences are not supporting details and detract from the main idea.
If students are able to identify the main idea of a passage, the effect is twofold. First, they know what the paragraph is about, and second, they are able to identify those sentences that work as supporting details--those details that add to the overall purpose of the paragraph. Those sentences that do not contribute to the overall effect of the paragraph are irrelevant and should be eliminated.