This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
偠 combines 亻 (person) with 要, which likely contributes the sound. The exact historical development is uncertain.
A person 亻 beside 要 (need/essential) might suggest a person who is essential or needed, but the actual meaning is unclear.
For ヨウ, imagine a person 亻 who is essential 要 and says "yo!" to get attention: yo -> ヨウ.