When is "devoir" used to mean "need to"?

Question

I am confused by the quizzes I am taking on when to use "devoir" and "avoir un besoin de" for "need to". The first question was: "How could you say 'You need to rest?'" I answered that both "Tu as besoin de repos" and "Tu dois de repos" are possible but was told only the first is correct. So, in the second test, when asked how to say "Marie needs to buy a new handbag", I answered only "Marie a un  besoin de ..." but was marked wrong for not ALSO choosing "Marie doit acheté..." Finally, on the third quiz, I was asked how to say "You need a new bike." In this case I chose both "avoir besoin de" and "devez..." but this time, like in the first question, I was told only the "avoir besoin de" is correct. I've studied the lesson several times. It says sometimes "devoir" can mean "need to" but it doesn't explain what those times are, and I cannot figure out any distinction in the three sentences above. Aidez moi, svp!

Answer

In the first test, the second answer would be: Tu dois te reposer.
In the second test:Marie a besoin d'un nouveau sac à main. -- Marie needs a new handbag.Marie doit acheter un nouveau sac à main. -- Marie must buy a new handbag.
You need a new bike. -- Vous avez besoin d'un nouveau vélo.You must buy a new bike. -- Vous devez acheter un nouveau vélo.
Avoir besoin de... -- "to have the need for something"Devoir faire quelque chose -- "to have to do something"