When the person has to express desire, he uses "want" or "would like" in his sentences. I like to distinguish between the fact that want is considered impolite by many and would like is more polite. Want is forceful and direct compared to using "would like", yet using the would does not necessarily mean you are indirect. Would grammatically speaking is used to express a conditional form of like and can be used elsewhere especially if there is an "if" in the sentence. Would is also the past form of will and I tell students not to mix the two up especially when will is used to express what somewhat is determined to do and would is used to offer something to someone as in asking what a person "would like".
When a person wants to express "need" it is because he cannot do without that object like we all need air to breathe. Need is often confused with "must" and "have to" which relate obligation. Must is what the person expects of himself as in I must leave at ten. Have to is generally used when the obligation is applied externally. When a person says he needs to go, he should keep in mind that he is referring to a situation that he can be without as opposed to saying he must go because he has given himself the obligation of doing so.
