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I've seen a learner in this forum who thought that "the topic" was something like out of the sentence, didn't have any function in the sentence and was just add there. Are you also thinking like this, maybe?
Check again the example sentences in the site I linked above. All the topics have function, and は can be replace with other particles in each sentence
You might think that you already know what is explained in Genki, but I believe it's worth reading. If you have questions in the book, you can ask it here, as many members do.