When does one need to use "lo/la"

Question

For example, you can say "compré el libro" and people would understand it just fine, but I've sometimes seen people say these kind of sentences like "Lo compré el libro". I think it still translates to "I bought the book", but what is the point of putting "lo/la" there and is it really necessary?

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Answer 1

"Lo compré el libro

This is redundant because the lo in this case is a pronoun that takes the place of a noun, so it should be Lo compré. Pronouns in these cases should only be used when context indicates their meanings.