The words knead, kneed, need sound the same but have different meanings and spellings. Why do knead, kneed, need sound the same even though they are completely different words?
The answer is simple: knead, kneed, need are homophones of the English language.
To mix and work into a uniform mass, as by folding, pressing, and stretching with the hands: kneading dough.
To make or shape by or as if by folding, pressing, and stretching with the hands.
To squeeze, press, or roll with the hands, as in massaging: kneading a painful calf muscle.
Having some specific type of knee or knees.
Simple past tense and past participle of knee.
A condition or situation in which something is required or wanted: crops in need of water; a need for affection.
Something required or wanted; a requisite: "Those of us who led the charge for these women's issues ... shared a common vision in the needs of women” ( Olympia Snowe).
Necessity; obligation: There is no need for you to go.
A condition of poverty or misfortune: The family is in dire need.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").