必(bì)须(xū) is an adverb and means ‘must’ while必(bì)需(xū) is an adjective and means ‘necessary’. Although the two words are same in pronunciation, they are different in morphemes. And the morpheme ‘需’ means ‘need’. For example,
维生素是我们必需的营养，所以你必须吃。
Wéishēngsù shì women bìxūde yíngyǎng， suǒyǐ nǐ bìxū chī
Vitamin is our necessary nutrition so you must eat
‘You must eat Vitamin as it is the necessary nutrition to us.’
