I question that also. (Need) is not equal to (have) is not equal to (must). Obviously in colloquial speech, people overlap the meanings/usage, but formally, they sometimes have some differences.
found reference:
Originally Posted by First Certificate Expert Coursebook By Jan Bell and Roger Gower, Longman - must is used to express strong, personal obligation or necessity:
I must post the letter straightaway. (I, personally, feel it is necessary)
- have (got) to is used when the situation or someone else (not the speaker) makes it necessary:
I have to work late tonight. My boss says so.
And other need/have/must thingys: надобно и надлежать, a verb with incomplete declension, impersonal only?.