"Why do we need sleep?"
Translation:Waarom hebben we slaap nodig?
'Nodig hebben' is transitive so it needs an object, in this case 'slaap' (without a preposition like 'van').
You are trying to translate word-for-word the phrase "having need for sleep", it seems. That doesn't work in Dutch, "nodig" is not a noun like the English word "need" can be. Instead, "nodig hebben" (corresponding to the English verb "to need") uses a direct object (in this case, "slaap") without any preposition.
A possible translation with the structure that you are try to use, would be:
"Waarom hebben we behoefte aan slaap?"
where "behoefte aan" translates as "a need for".
Thanks. I was probably thinking French (avoir besoin de). English doesn't normally use the word "have" at all in this case.
can someone please post the answer with "behoefte"? my answer was wrong but the correct answer that appears at the bottom is only with the "nodig" part. or is it the case that "behoefte" can't be used?